NM: Application Error - Object reference not set to an instance of an object. (4313864)

Issue

A popup window is displayed with the following: Object reference not set to an instance of an object. 

Solution

The NAB user does not contain a Shortname
or
The AD account used for migrations is logged into more than 5 workstations.
Make sure you have not exceeded the maximum number of accounts concurrently logged into the Exchange server.  An AD account should not be used for more than 5 migration workstation. Example: If you have 15 migration workstations, you should have at least 3 migrations accounts.