[Error] Trainer partner pokémon switching error

So I have this problem when I have a trainer as a partner for double battles. When the trainer's pokémon faints, instead of sending out another pokémon it shows the following error.
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Exception: NoMethodError

Message: undefined method `ability' for nil:NilClass

PokeBattle_Battle:1551:in `pbSwitch'

PokeBattle_Battle:1544:in `each'

PokeBattle_Battle:1544:in `pbSwitch'

PokeBattle_Battle:4117:in `__clauses__pbEndOfRoundPhase'

PokeBattle_Clauses:42:in `pbEndOfRoundPhase'

PokeBattle_DynamicBattle:202:in `pbStartBattleCore'

PokeBattle_DynamicBattle:201:in `logonerr'

PokeBattle_DynamicBattle:201:in `pbStartBattleCore'

PokeBattle_DynamicBattle:185:in `loop'

PokeBattle_DynamicBattle:206:in `pbStartBattleCore'

That happens always at the end of each turn after the ally trainer's pokémon faints.