A null parameter has been passed

Hi all

I'm facing a problem when I try run this code:

a!richTextDisplayField(
labelPosition: "COLLAPSED",
value: {
a!richTextItem(
text: {
hour(ri!ES_Working_Hours_Record.LeavingTime - ri!ES_Working_Hours_Record.StartTime),
" hours",
if(
hour(ri!ES_Working_Hours_Record.LeavingTime - ri!ES_Working_Hours_Record.StartTime) = "8",

"
Working hours are complete",
if(
and(8 > hour(ri!ES_Working_Hours_Record.LeavingTime - ri!ES_Working_Hours_Record.StartTime) > 0),
"
Working hours are not complete",
if(
hour(ri!ES_Working_Hours_Record.LeavingTime - ri!ES_Working_Hours_Record.StartTime) = 0,
" insert working hours correctly please",
" working hours passed limit"
)
)
)
},
size: "STANDARD",
style: {
"STRONG"
}
)
}
)
},

I got this message: Error Evaluating UI Expression Expression evaluation error [evaluation ID = ] in rule 'es_working_hours_record2' at function 'hour' [line 33]: A null parameter has been passed.

I need to make it null, because I want the user who inserts these values, so I think it should be Null.