Error while deleting multiple user from grid

a!localVariables(
  local!selectionValue,
  local!isUserSelected: runtimetypeof(local!selectionValue) = 'type!{http://www.appian.com/ae/types/2009}User',
  local!actionTaken: if(
    isnull(ri!btnAction),
    false,
    or(ri!btnAction = "ADD")
  ), 
  local!navigationLength: if(
    isnull(ri!navigationPath),
    0,
    length(ri!navigationPath)
  ),
  a!formLayout(
    label: "User Management",
    contents: {
      a!pickerFieldGroups(
        label: "Select a group to view its members",
        maxSelections: 1,
        value: ri!initialGroup,
        saveInto: {
          ri!initialGroup,
          a!save(
            {
              ri!btnAction,
              ri!addedToGroup,
              ri!removeFromGroup,
              ri!navigationPath,
              local!selectionValue
            },
            null
          )
        }
      ),
      a!sectionLayout(
        label: group(ri!initialGroup, "groupName") & " Members",
        contents: {
          a!buttonLayout(
              a!buttonWidget(
                label: "Remove Member",
                value: a!removeGroupMembers(
                  members:ri!removeFromGroup
                ),
                saveInto: {
                  ri!btnAction,
                  a!save(ri!userToRemove, local!selectionValue),
                  /*
                    Since a user needs to be removed from a group, the button
                    needs to determine what group the user should be removed from.
                  */
                  a!save(
                    ri!removeFromGroup,
                    if(
                      /* If the user is on the first column, save the initial group */
                      local!navigationLength = 1,
                      ri!initialGroup,
                      /* Otherwise save the group containing the selected user */ 
                      ri!navigationPath[local!navigationLength - 1]
                    )
                  ),
                  /*
                    This navigation path will be used to pre-populate the group browser
                    when the user returns to this interface after the selected user has
                    been removed. Therefore, we must remove the selected user from the
                    navigation path to prevent an error.
                  */
                  a!save(ri!navigationPath, rdrop(ri!navigationPath, 1))
                },
                submit: true,
                color: "NEGATIVE",
                disabled: or(ri!btnAction,(local!selectionValue)),
              )
          ),
          a!sectionLayout(    
            label: "",
            contents: {
              a!gridField(
                labelPosition: "COLLAPSED",
                data: a!groupMembers(
                  group: ri!initialGroup
                ),
                columns: {
                  a!gridColumn(
                    label: "Group Members",
                    value: fv!row,
                    sortField: "displayName"
                  )
                },
                pagingSaveInto: null,
                borderStyle: "STANDARD",
                selectable: true,
                selectionStyle: "CHECKBOX",
                selectionValue: local!selectionValue,
                selectionSaveInto: local!selectionValue
              )
            }
          ),
          a!linkField(
            labelPosition: "COLLAPSED",
            links: {
              a!dynamicLink(
                label: "+ Add Users to " & group(ri!initialGroup, "groupName"),
                value: "ADD",
                saveInto: {
                  ri!btnAction,
                  a!save(ri!addedToGroup, ri!initialGroup)
                }
              )
            },
            showWhen: not( local!actionTaken)
          )
        },
        showWhen: not(isnull(ri!initialGroup))
      ),
      a!sectionLayout(
        label: "Add Users to " & group(ri!addedToGroup, "groupName"),
        contents: {
          a!pickerFieldUsers(
            label: "Users to Add",
            value: ri!usersToAdd,
            saveInto: a!save(ri!usersToAdd, getdistinctusers(save!value)),
            required: true
          )
        },
        showWhen: ri!btnAction = "ADD"
      ),
      a!buttonLayout(
        primaryButtons: {
          a!buttonWidget(
            label: if(ri!btnAction = "ADD", "Add Users", ""),
            submit: true,
            style: "SOLID"
          )
        },
        secondaryButtons: {
          /*
            Allows the user to cancel the selected action. If the user
            cancels out of the action, we need to clear all the
            selection variables
          */
          a!buttonWidget(
            label: "Cancel",
            value: null,
            saveInto: {
              ri!btnAction,
              ri!userToRemove,
              ri!addedToGroup,
              ri!removeFromGroup,
              ri!usersToAdd,
              local!selectionValue
            },
            style: "OUTLINE",
            showWhen: local!actionTaken
          )
        },
        showWhen: local!actionTaken
      )
    },
    buttons: a!buttonLayout(
      primaryButtons: {
        a!buttonWidget(
          label: "Close",
          value: "CLOSE",
          saveInto: ri!btnAction,
          submit: true,
          validate: false
        )
      }
    )
  )
)