The Test Data Transfer Tool can be invaluable in copying data from one environment to another for testing. Sometimes you may want to export the data against a restored copy of an environment, either due to security, or to limit any chance of performance degradation during the export.
It is important that you make a copy of both the AX data database as well as the model. The tool exports table and field ID information from the model database. If that model database does not exist with the right name, you will get 'object reference not set to an instance of an object' errors in the export log:
C:\Program Files (x86)\Microsoft Dynamics AX 2012 Test Data Transfer Tool (Beta)\DPLog.xml