Error: Failed to access bus output set in data base. System Topology not defined.

Solution:
Project validation will fix this issue. To validate the project, save and close the problematic project. Within the PTW32 software, click on the Project menu > Open and navigate to the project folder. While holding down both 'Ctrl' and 'Shift' buttons on the keyboard, open the .prj file. It will now ask you to validate the database. Select 'OK' and let the process finish. Any problems found during this process will automatically be fixed.