Problem Saving values of a dropdown inside an editable grid

Hi! I´m trying to create a dropdown field inside an editable grid, based on a list

I have created the following list (the choice labels and choice values appear correctly)

But the problem appears when I try to add the dropdown field value: 

Interface Definition: Expression evaluation error [evaluation ID = 02S6W] : An error occurred while executing a save: Expression evaluation error: You must specify a variable to save into, such as ri!name << fn!sum. Received: Text.

My code is the following (problem located around line 71):

 a!sectionLayout(
      label: "Líneas Factura",
      labelIcon: "list-alt-solid",
      contents: {
        a!gridLayout(
          label: "A continuación, incorpore las líneas de factura correspondientes y pulse el botón de creación",
          headerCells: {
            a!gridLayoutHeaderCell(label: "Linea Factura"),
            a!gridLayoutHeaderCell(label: "Factura"),
            a!gridLayoutHeaderCell(label: "Pedido"),
            a!gridLayoutHeaderCell(label: "Proveedor"),
            a!gridLayoutHeaderCell(label: "Material"),
            a!gridLayoutHeaderCell(label: "Precio Unitario"),
            a!gridLayoutHeaderCell(label: "Cantidad"),
            a!gridLayoutHeaderCell(label: "Importe Total"),
            a!gridLayoutHeaderCell()
          },
          columnConfigs: {
            a!gridLayoutColumnConfig(width: "DISTRIBUTE"),
            a!gridLayoutColumnConfig(width: "DISTRIBUTE"),
            a!gridLayoutColumnConfig(width: "DISTRIBUTE"),
            a!gridLayoutColumnConfig(width: "DISTRIBUTE"),
            a!gridLayoutColumnConfig(width: "DISTRIBUTE"),
            a!gridLayoutColumnConfig(width: "DISTRIBUTE"),
            a!gridLayoutColumnConfig(width: "DISTRIBUTE"),
            a!gridLayoutColumnConfig(width: "DISTRIBUTE"),
            a!gridLayoutColumnConfig(width: "ICON")
          },
          rows: a!forEach(
            items: local!LineasFactura,
            expression: a!gridRowLayout(
              contents: {
                a!textField(
                  value: fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{181742a7-4305-4134-b05e-4eebc9c56292}idLineaFactura'],
                  saveInto: {
                    fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{181742a7-4305-4134-b05e-4eebc9c56292}idLineaFactura']
                  }
                ),
                a!textField(
                  value: fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{9c440183-9c71-423b-a300-eb4c3ea747fb}idFactura'],
                  saveInto: {
                    fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{9c440183-9c71-423b-a300-eb4c3ea747fb}idFactura']
                  }
                ),
                a!textField(
                  value: fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{60774c8e-9b47-486f-9fea-b32c525c05fd}idPedido'],
                  saveInto: {
                    fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{60774c8e-9b47-486f-9fea-b32c525c05fd}idPedido']
                  }
                ),
                a!textField(
                  value: fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{844be35b-6de5-434e-9c69-e7c7b8a148dd}proveedor'],
                  saveInto: {
                    fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{844be35b-6de5-434e-9c69-e7c7b8a148dd}proveedor']
                  }
                ),
                a!dropdownField(
                  label: "Proveedor",
                  labelPosition: "ABOVE",
                  placeholder: "--- Seleccione el Material ---",
                  choiceLabels: local!ListadoMateriales,
                  choiceValues: local!ListadoMateriales,
                  value: fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{019eb28b-727b-4e2c-8160-1bcf8a747ef1}material'],
                  saveInto: { fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{019eb28b-727b-4e2c-8160-1bcf8a747ef1}material'],
                  searchDisplay: "AUTO",
                
                
                  }
                ),
                a!floatingPointField(
                  value: fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit'],
                  saveInto: {
                    fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit'],
                    a!save(
                      fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{53f231a2-9bf7-40a1-af07-74373fa7bd9f}importe'],
                      if(
                        or(
                          isnull(
                            fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad']
                          ),
                          isnull(
                            fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit']
                          )
                        ),
                        0,
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad'] * fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit']
                      )
                    )
                  },
                  required: true,
                  validations: {
                    if(
                      a!isNotNullOrEmpty(
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit']
                      ) = false,
                      "",
                      if(
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit'] < 0,
                        "El precio unitario debe ser superior a cero",
                        ""
                      )
                    )
                  }
                ),
                a!integerField(
                  value: fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad'],
                  saveInto: {
                    fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad'],
                    a!save(
                      fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{53f231a2-9bf7-40a1-af07-74373fa7bd9f}importe'],
                      if(
                        or(
                          isnull(
                            fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad']
                          ),
                          isnull(
                            fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit']
                          )
                        ),
                        0,
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad'] * fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit']
                      )
                    )
                  },
                  required: true,
                  validations: {
                    if(
                      a!isNotNullOrEmpty(
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad']
                      ) = false,
                      "",
                      if(
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad'] < 1,
                        "La cantidad debe ser superior a cero",
                        ""
                      )
                    )
                  }
                ),
                a!floatingPointField(
                  value: if(
                    or(
                      isnull(
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad']
                      ),
                      isnull(
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit']
                      )
                    ),
                    0,
                    fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{b1b5fc0b-5be4-45be-8342-381cdc56017d}cantidad'] * fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{eda9bda7-ae57-44e7-a651-d097c8581021}precioUnit']
                  ),
                  saveInto: {
                    fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{53f231a2-9bf7-40a1-af07-74373fa7bd9f}importe']
                  },
                  required: true,
                  validations: {
                    if(
                      a!isNotNullOrEmpty(
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{53f231a2-9bf7-40a1-af07-74373fa7bd9f}importe']
                      ) = false,
                      "",
                      if(
                        fv!item['recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{53f231a2-9bf7-40a1-af07-74373fa7bd9f}importe'] < 0,
                        "El importe debe ser superior a cero",
                        ""
                      )
                    )
                  },
                  align: "RIGHT"
                ),
                a!richTextDisplayField(
                  value: {
                    a!richTextIcon(
                      icon: "times-circle",
                      link: a!dynamicLink(
                        value: remove(local!LineasFactura, fv!index),
                        saveInto: local!LineasFactura
                      ),
                      linkstyle: "STANDALONE",
                      color: "NEGATIVE"
                    )
                  }
                )
              }
            )
          ),
          addrowlink: a!dynamicLink(
            label: "Añadir nueva Línea de Factura",
            saveInto: a!save(
              target: local!LineasFactura,
              value: append(
                local!LineasFactura,
                'recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura'(
                  'recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{9c440183-9c71-423b-a300-eb4c3ea747fb}idFactura': ri!recordFact['recordType!{51d44df6-c727-4443-8114-aef8a7509b41}CPF Maestro Facturas.fields.{6f993c32-7f64-424a-859f-d779194c4a18}id_factura'],
                  'recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{60774c8e-9b47-486f-9fea-b32c525c05fd}idPedido': ri!recordFact['recordType!{51d44df6-c727-4443-8114-aef8a7509b41}CPF Maestro Facturas.fields.{e8009662-ca1c-43da-9c72-2840463214be}id_pedido'],
                  'recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{844be35b-6de5-434e-9c69-e7c7b8a148dd}proveedor': ri!recordFact['recordType!{51d44df6-c727-4443-8114-aef8a7509b41}CPF Maestro Facturas.fields.{c2a60669-567e-41b8-acdd-29729912b3e5}nomProveedor'],
                  'recordType!{6e637180-b344-49d2-a1f7-f87eed0041ba}CPF Lineas Factura.fields.{181742a7-4305-4134-b05e-4eebc9c56292}idLineaFactura': count({local!LineasFactura}) + 1
                )
              )
            )
          )
        )
      }
    ),
    a!buttonLayout(
      primaryButtons: {
        a!buttonWidget(
          label: "Completar Creación Factura",
          saveInto: a!save(ri!recordLineasFact, local!LineasFactura
          ),
          submit: true,
          style: "PRIMARY",
          validate: true
        )
      },
      secondaryButtons: {
        a!buttonWidget(
          label: "Cancelar",
          value: true,
          saveInto: ri!cancel2,
          submit: true,
          style: "NORMAL",
          validate: false
        )
      }
    )
  }
)

Thanks a lot!