This error can occur for several reasons. I have also encountered it, but it is very difficult to fix. Reasons for this error:
1) There was a bug with the game engine, more specifically with the in-game screens and the RPG framework.
2) A dialogue screen was called up, which you exited incorrectly, and this error will now occur constantly.
3) Perhaps you used cheats or exploits and broke the game's logic.
If you can identify the exact cause, you will help to fix it.