Symptoms:
While transferring time entries from Web TimeSheet to QuickBooks, the Integration Manager throws the following error message when clicking the Transfer button,

"Unhandled exception has occurred in application. System null.reference.exception: Object reference not set to an instance of an object"

Cause:
This error is usually seen when the mapping includes class and the wrong mapping has been provided as the source in Web Timesheet to map to the class in QuickBooks. For example: If the department code is created as class in Quick Books and in Web TimeSheet the department code is a user defined field on Timesheet row level, then if the mapping is wrongly set as Department:Department Code as Class in QuickBooks the above error is displayed. The correct mapping should have been Timesheet:Department Code as Class in QuickBooks

Resolution:
Identify the correct class mapping that corresponds to the field in Web TimeSheet. Also, if the class mapping is not necessary, you can leave the mapping as not selected and proceed with the time transfer.