getting error please help

Try this

a!forEach(
  pv!addedResearchItems_cdt,
  if(
    and(
      rule!APN_isBlank(index(fv!item, "id", {})),
      rule!APN_isBlank(fv!item.url)
    ),
    cast(
      'type!{urn:com:appian:types:gngp}GNGP_RESEARCH_ITEM',
      a!update(
        fv!item,
        { "file_name" },
        {
          if(
            not(isnull(fv!item.file_name)),
            fv!item.file_name,
            if(
              not(isnull(fv!item.document_id)),
              document(todocument(fv!item.document_id), "name") & "." & document(
                todocument(fv!item.document_id),
                "extension"
              ),
              {}
            )
          )
        }
      )
    ),
    fv!item
  )
)

getting below error

(Expression evaluation error at function a!forEach: Error in a!forEach() expression during iteration 1: Expression evaluation error at function a!update [line 5]: Cannot update GNGP_RESEARCH_ITEM; received 1 keys and 0 values) (Data Outputs)

Make sure code returns a value to update or use a null check before
if(
not(isnull(fv!item.file_name)),
fv!item.file_name,
if(
not(isnull(fv!item.document_id)),
document(todocument(fv!item.document_id), "name") & "." & document(
todocument(fv!item.document_id),
"extension"
),
{}
)
)