What does this NoMethodError mean?

[MrJediMan]

What does this message mean?
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Pokemon Essentials
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[Pokémon Essentials version 17.1]
Exception: NoMethodError
Message: undefined method `pokedexSeen' for nil:NilClass
PSystem_Utilities:992:in `pbSetViableDexes'
PSystem_Utilities:989:in `each'
PSystem_Utilities:989:in `pbSetViableDexes'
PScreen_PauseMenu:92:in `pbStartPokemonMenu'
Scene_Map:127:in `call_menu'
Scene_Map:213:in `update'
Scene_Map:234:in `main'
Scene_Map:231:in `loop'
Scene_Map:236:in `main'
Main:49:in `mainFunctionDebug'

This exception was logged in
C:\Users\saulu\Saved Games\Pokemon Essentials\errorlog.txt.
Press Ctrl+C to copy this message to the clipboard.
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OK
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[Fontbane]

You must've accidentally deleted the line that defines the "pokedexSeen" method. If you have a clean copy of essentials to compare your scripts to (I recommend this if you don't already have it), ctrl+f for "pokedexSeen" in the scripts to see where it's defined. I could totally be wrong though.

 

[Vendily]

Pokedexseen is a method of pokebattle_trainer. If this is nil, that means you did not set up your trainer event correctly.
Look st the example maps, specifically intro, to see how it is supposed to be done.