[Error] What's causing this "Trick Room When Sent Out" Ability code to crash?

[SuperSpyroDragon64]

BattleHandlers::AbilityOnSwitchIn.add(:TRICKSTER,
  proc { |ability,battler,battle|
      @battle.field.effects[PBEffects::TrickRoom] = 5
      @battle.pbDisplay(_INTL("{1} twisted the dimensions!",user.pbThis))
  }
)

This code should work. Probably. But instead, when a Pokemon with this ability is sent out, the game crashes and the error message says this:

[Pokémon Essentials version 19.1]
[Generation 8 Project v1.1.0]
[v19.1 Hotfixes 1.0.7]

Exception: NoMethodError
Message: undefined method `field' for nil:NilClass

Backtrace:
142:BattleHandlers_Abilities:2840:in `block in <main>'
035:Event_Handlers:199:in `trigger'
141:BattleHandlers:454:in `triggerAbilityOnSwitchIn'
154:Battler_AbilityAndItem:14:in `pbEffectsOnSwitchIn'
176:Battle_Action_Switching:312:in `block in pbOnActiveAll'
176:Battle_Action_Switching:312:in `each'
176:Battle_Action_Switching:312:in `pbOnActiveAll'
173:Battle_StartAndEnd:301:in `pbStartBattleCore'
173:Battle_StartAndEnd:258:in `pbStartBattle'
230:Overworld_BattleStarting:291:in `block (2 levels) in pbWildBattleCore'

Why does this happen? How can it be fixed?

 

[Luka S.J.]

Notice the difference between the battle that you have in the proc definition and @battle you're trying to use. You don't have access to an instance variable @battle in that code context. Use the battle variable that comes from the proc arguments.

 

[SuperSpyroDragon64]

This works now, anyone who wants to use it is free to use it.

BattleHandlers::AbilityOnSwitchIn.add(:TRICKSTER,
  proc { |ability,battler,battle|
      battle.field.effects[PBEffects::TrickRoom] = 5
      battle.pbDisplay(_INTL("{1} twisted the dimensions for 5 turns!",battler.pbThis))
  }
)