Here is the source code for BaseSimpleGame: https://jme.dev.java.net/source/browse/checkout/jme/src/com/jme/app/BaseSimpleGame.java?rev=1.27.
In the cleanup() function, this command is called:
display.getRenderer().cleanup();
Unfortunately, Renderer does not seem to have the cleanup() function available, and it is necessary that I call it, since I chose not to extend any of the game classes that came with jME. Can anyone tell me how to clean up the renderer, so that the game can run properly?