What does this mean and how can I fix it?

[Rayquaza.]

This error message keeps showing up on random encounters in grass or in a cave or any wild pokémon battles. Please can someone tell me what needs fixing?

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Exception: NoMethodError
Message: undefined method `trainertype' for nil:NilClass
PokeBattle_Battle:2667:in `pbSendOut'
PokeBattle_Battle:727:in `pbStartBattleCore'
PokeBattle_Battle:515:in `pbStartBattle'
PokemonField:932:in `pbWildBattle'
PokemonField:931:in `pbSceneStandby'
PokemonField:933:in `pbWildBattle'
PokemonField:930:in `pbBattleAnimation'
PokemonField:930:in `pbWildBattle'
PokemonField:1337:in `pbBattleOnStepTaken'
PokemonField:1361:in `pbOnStepTaken'
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[FL]

I guess that the script can't find the trainer type of the player, something like 'POKEMONTRAINER_Red'. Do a test with a different player.