Cobblemon Trainer Battle

• Fixed throwing ConcurrentModificationException when iterating over players in BattleFledEventHandler
• Fixed throwing NullPointerException when executing commands
• (Hopefully) fixed throwing Javascript TypeError (RuntimeException in Java) when players teleport away from Pokemon battle

Regarding the third issue, the exact cause is unknown but it's probably due to calling PokemonBattle#writeShowdownAction() before calling PokemonBattle#end(), causing race condition with PokemonBattle#tick() which also try to call PokemonBattle#writeShowdownAction()