Sounds like the Plan G install went bad. Cause be your virus scanner or tried to install to a windows protected folder. "The Object reference not set to an instance of an object" is a bad value in the code or file which has caused a bad or missing reference: this is referred to as a "NULL" value or reference.
(That error will also occur if you have have instaled an older version of Plan G IE: v3.1.4 or v3.2.1 and are trying to update the navigation database for P3Dv4. (v3.2.2 is required)
Anyways: try the following:
1. Uninstall Plan G.
2, Turn off your virus scanner or blockers.
3. Re-Install the latest version of PLAN G. Install as admin and install to a location that is outside any windows protected folder.
4. Once done - set the program to always Run as administrator. (right click on the icon > properties- and go to compatibility tab - Check "run as administrator".
5. Once that is done, Run the Plan G program-
Go to the Data tab > Build Navigation Database, select the correct sim database, then select the settings tab: You have to navigate to the correct locations for your sim and scenery.cfg file (and add any additional scenery locations). Then click OK to save the information - On the next screen, click OK to execute the building of the navigation database.
Ps: Note: (*Right now the current Plan G version will not generate the navigation database for P3Dv5 )
Norm