Check if I have a document Existed or not?

Hello folks ,

I am getting this below error while displaying the interface 

Error Evaluating UI Expression Expression evaluation error [evaluation ID = EEW8HICE] in rule 'User_employer' (called by rule 'User_programsummary') at function a!forEach [line 501]: Error in a!forEach() expression during iteration 10: Expression evaluation error in rule 'utils_returndocumentpath' at function fn!substitute [line 9]: A null parameter has been passed as parameter 1. (APNX-1-4198-000).

Code in the rule is "utils_returndocumentpath"

fn!substitute(
if(or(isnull(ri!doc),ri!doc="",length(ri!doc)<0),"",if(and(not(isnull(ri!doc)), fn!isobjectexists("Document", ri!doc)), document(ri!doc,"name")&"."&document(ri!doc,"extension"), null)),
"/",
"_"
)

Code at line 501 :

a!gridTextColumn(
label:" Acceptance Agreement",
field:"empAccpDocID",
data:a!forEach(
items:index(local!employers.data,"empAccpDocID",null),
expression:if(or(isnull(fv!item),fv!item=""),null,rule!UTILS_returnDocumentPath(fv!item))

),
links:a!forEach(
items:index(local!employers.data,"empAccpDocID",null),
expression:if(isnull(fv!item),null,a!documentDownloadLink(document: todocument(fv!item)))

)

)

Thanks,

Sandeep