visa approval rule null error

We have a request for Visa Approval in personal information

Where employees from Canada and US if they made changes in VISA like add , edit , delete it has to go for special role for approval and other countries should get auto approval.

when we are trying to delete we are getting null error - An attribute in the US_CA_VISA rule has a null value.

rules are

for Canada and US for special role approval we have given (US_CA_VISA)

((Requestor's Assignment.result.personNumber == Worker's Assignment.result.personNumber) AND (Proposed Person Visa Permit.result.IssuingCountryName in "United States","Canada") AND (Proposed Person Visa Permit != null))