The warning message is because a Payment File has a null/blank status. This status is caused by deleting a payment file from the list. To resolve this:
1. Navigate to Payments > Payment Processing > Payment File Administration.
2. Click on Edit this Search.
3. Under Criteria tab > Standard subtab > remove the File Processed? filter.
4. Click Preview.
5. Edit the payment file where the Payment File Processed? column is blank/null.
6. Set the File Processed? to Cancelled.
7. Click Save.
NOTE: To prevent this from happening again, if a payment is to be cancelled, instead of deleting, reverse the payment file instead.