Showing and hiding a section

I am trying to achieve a functionality where a user upload an excel then added a function to check the format of the excel file whether it matches with cdt or not. if matches, then user can proceed otherwise a hidden section layout will appear with a warning and download option for correct excel sample format but i am getting error in readexcelsheetpaging() as it shows null value error for document parameter.i try to fix it by using if condition and it solves the null error but now when i try to upload an excel file it shows other error "Interface Definition: Expression evaluation error at function 'readexcelsheetpaging' [line 2]: com.appiancorp.suiteapi.content.exceptions.InvalidContentException: Invalid Content ID".

a!localVariables(
  local!excelFormatFromFile: readexcelsheetpaging(
    excelDocument: if(
      isnull(ri!excelFile),
      cons!SD_EXCELFILE,
      ri!excelFile
    ),
    sheetNumber: 0,
    paginginfo: a!pagingInfo(1, 1),
    numberOfColumns: 11
  ).data.values[1],
  local!cdt: a!keys(
    value: 'type!{urn:com:appian:SD}SD_Student_Data'()
  ),
  local!excelFormatCheck: and(
    a!forEach(
      items: local!excelFormatFromFile,
      expression: {
        if(
          local!cdt[fv!index + 1] = local!excelFormatFromFile[fv!index],
          true,
          false
        )
      }
    )
  ),
  a!formLayout(
    label: "Excel File Upload",
    contents: {
      a!sectionLayout(
        contents: {
          a!fileUploadField(
            label: "File Upload",
            labelPosition: "ABOVE",
            target: cons!SD_CONSTANT_DOCUMENT,
            fileNames: "Excel File" & now(),
            maxSelections: 1,
            value: ri!excelFile,
            saveInto: ri!excelFile,
            validations: {
              a!localVariables(
                local!invalidExtensions: difference(
                  upper(fv!files.extension),
                  { "XLSX", "XLS" }
                ),
                if(
                  length(local!invalidExtensions) > 0,
                  "Invalid File Format.File format should be of xls or xlsx ",
                  ""
                )
              ),
              
            }
          ),
          a!sectionLayout(
            label: "Invalid excel file format.Refer to this document for excel sheet format",
            contents: {
              a!linkField(
                label: "",
                labelPosition: "ABOVE",
                links: {
                  a!documentDownloadLink(
                    document: cons!SD_EXCELFILE,
                    label: "Download Sample Document"
                  )
                }
              )
            },
            showWhen: { local!excelFormatCheck = false }
          )
        }
      )
    },
    buttons: a!buttonLayout(
      primaryButtons: {
        a!buttonWidget(
          label: "Submit",
          submit: true,
          style: "PRIMARY"
        )
      },
      secondaryButtons: {
        a!buttonWidget(
          label: "Cancel",
          value: true,
          saveInto: ri!cancel,
          submit: true,
          style: "NORMAL",
          validate: false
        )
      }
    )
  ),
  
)