The following error message appears, in a custom column cell when generating a report.

"Null reference"

Web TimeSheet detects most formula errors when the formula is saved. However, some errors called run-time errors are indicated by an error code displaying in the report cell after the report is generated.

The Null reference error code suggests that the formula includes a reference to another column that has no value hence, identify the other column and that does not have value to fix it.