I commented out some lines in the code and it works now. But it is very buggy at the moment. The models are looking quite good but the clipping is much to early and too hard. I made the experience that it improves the performence a lot if you put the grass into one big model. Shadows are missing
The level itself isn´t good designed as well. Make sure that even if it is small the player has the feeling as if it is a big world and not just this one terrain.
-> Place mountains everywhere around your level.