FYI, I've investigated and I am 100% positive that the rumors implicating Railcraft in this crash are false. I just don't see any way my Pumpkin Cart could cause that crash. Its a completely different code path.
I agree with ConvertJaguar. I have read that setting the drop chance to -1 will causes a crash because it is out of range, but since proven that is not true.
I am pretty sure the issue is that dropChances has to be set to an instance of the object type Field before it can be called. This would be why it is throwing a java.lang.NullPointerException.