How to connect a data store to a record

Thanks for your answer! The fact is that I had to make an editable grid with the function append in order to add new rows. I followed a tutorial and they used data store in order to make the local variables. I would prefer to do everything only with records, as you said before, but I´m new in appian and, even though I tried to substitute everything and use a record, I have problems in some lines, specially in the function append. 

How could I substitute the array for the record in my code? Any help would be useful, thank you so much!

I show you the editable grid code so you can understand my problem:

a!localVariables(
local!gridDataType: 'type!{urn:com:appian:types:PTPP}PTPP_INVOICE_LINES'(),

a!formLayout(
label: "Invoice Detail",
contents: {
a!gridLayout(
label: "Invoice Lines Information",
labelPosition: "ABOVE",
headerCells: {
a!gridLayoutHeaderCell(label: "Purchase Order"),
a!gridLayoutHeaderCell(label: "Date"),
a!gridLayoutHeaderCell(label: "Total Value"),
a!gridLayoutHeaderCell(label: "Material"),
a!gridLayoutHeaderCell(label: "Supplier"),
a!gridLayoutHeaderCell(label: "")
},
columnConfigs: {
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: ri!Array_InvoiceLines,
expression: a!gridRowLayout(
contents: {
a!textField(value:fv!item.idOrden, saveInto: fv!item.idOrden),
a!textField(value:fv!item.fechaOrden, saveInto: fv!item.fechaOrden),
a!textField(value:fv!item.totalOrden, saveInto: fv!item.totalOrden),
a!textField(value:fv!item.material, saveInto: fv!item.material),
a!textField(value:fv!item.idProveedor, saveInto: fv!item.idProveedor),
a!richTextDisplayField(
value: a!richTextIcon(
icon: "times-circle",
color: "NEGATIVE",
link: a!dynamicLink(
saveinto: a!save(
target: ri!Array_InvoiceLines, value: remove(
ri!Array_InvoiceLines, fv!index
)
)
)
)
)
}
)
)

},
selectionSaveInto: {},
addRowLink: a!dynamicLink(
label: "Add new Invoice Line",
saveinto: a!save(target: ri!Array_InvoiceLines, append(ri!Array_InvoiceLines, local!gridDataType))
),
validations: {},
shadeAlternateRows: true
)
},
buttons: a!buttonLayout(
primaryButtons: {
a!buttonWidget(
label: "Submit ",
submit: true,
style: "PRIMARY",
validate: true
)
},
secondaryButtons: {
a!buttonWidget(
label: "Cancel",
value: true,
saveInto: ri!cancel,
submit: true,
style: "NORMAL",
validate: false
)
}
)
)
)

When I tried to use records, an error appears (cannot index null values). It´s true that I created my record from scratch and it´s empty, but I don´t think this should be a problem. If it is not an inconvenience, could you tell me what is wrong with my code ? I´m posting images and the real code, ( "PTPP_Invoice_Lines" is my empty record, I want the user to be able to add new rows of information, and update the record) and an image of the error too 

Next, error that happens when I try to add a new row (using records in the code)

Then, my code:

a!formLayout(
label: "Invoice Detail",
contents: {
a!gridLayout(
label: "Invoice Lines Information",
labelPosition: "ABOVE",
headerCells: {
a!gridLayoutHeaderCell(label: "Purchase Order"),
a!gridLayoutHeaderCell(label: "Date"),
a!gridLayoutHeaderCell(label: "Total Value"),
a!gridLayoutHeaderCell(label: "Material"),
a!gridLayoutHeaderCell(label: "Supplier"),
a!gridLayoutHeaderCell(label: "")
},
columnConfigs: {
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: ri!InvoiceLines,
expression: a!gridRowLayout(
contents: {
a!textField(value:ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{ab97d392-b365-425a-a0e5-116da756c87a}id_orden'], saveInto: ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{ab97d392-b365-425a-a0e5-116da756c87a}id_orden']),
a!textField(value:ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{25003162-d3d0-4466-86f8-471cb1e37589}fecha_orden'], saveInto: ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{25003162-d3d0-4466-86f8-471cb1e37589}fecha_orden']),
a!textField(value:ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{c228ed4c-21cb-4af4-a313-88f26e7874da}total_orden'], saveInto: ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{c228ed4c-21cb-4af4-a313-88f26e7874da}total_orden']),
a!textField(value:ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{5f4060f6-827d-41b5-88e7-0328ec1309bc}Material'], saveInto: ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{5f4060f6-827d-41b5-88e7-0328ec1309bc}Material']),
a!textField(value:ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{d9317466-7426-47af-bd64-22a998c636e6}id_proveedor'], saveInto: ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{d9317466-7426-47af-bd64-22a998c636e6}id_proveedor']),
a!richTextDisplayField(
value: a!richTextIcon(
icon: "times-circle",
link: a!dynamicLink(
saveinto: a!save(
target: ri!InvoiceLines, value: remove(
ri!InvoiceLines, fv!index
)
)
),
color: "NEGATIVE"
)
)
}
)
)

},
selectionSaveInto: {},
addRowLink: a!dynamicLink(
label: "Add new Invoice Line",
saveinto: a!save(target: ri!InvoiceLines, append(ri!InvoiceLines['recordType!{d269796e-dcd7-46b6-8ecf-55ed16bf54a1}PTPP Invoice Lines.fields.{ab97d392-b365-425a-a0e5-116da756c87a}id_orden'], ri!InvoiceLines))
),
validations: {},
shadeAlternateRows: true
)
},
buttons: a!buttonLayout(
primaryButtons: {
a!buttonWidget(
label: "Submit ",
submit: true,
style: "PRIMARY",
validate: true
)
},
secondaryButtons: {
a!buttonWidget(
label: "Cancel",
value: true,
saveInto: ri!cancel,
submit: true,
style: "NORMAL",
validate: false
)
}
)
)