Error going from OverWorld to BattleWorld

If I press space while next to the opponent, I am supposed to enter a
BattleWorld where the battle can start, but I get an error when I try to
and the game crashes. It's a Null Pointer Exception on Line 122 of
BattleWorld, but the trainer and opponent are there in OverWorld and is
supposed to pass it into the BattleWorld constructor. Where's the error
coming from?