[Error] Pokedex Error

I can't open Pokedex, I don't know why? Can someone help me? I also looked for solutions like below but still doesn't work.

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My error:
[Pokémon Essentials version 18.1]

Exception: NoMethodError

Message: undefined method `[]' for nil:NilClass

Backtrace:

PScreen_PokedexMain:334:in `pbGetDexList'

PScreen_PokedexMain:325:in `each'

PScreen_PokedexMain:325:in `pbGetDexList'

PScreen_PokedexMain:349:in `pbRefreshDexList'

PScreen_PokedexMain:260:in `pbStartScene'

PScreen_PokedexMain:1190:in `pbStartScreen'

PScreen_PauseMenu:161:in `pbStartPokemonMenu'

PScreen_PauseMenu:158:in `pbFadeOutIn'

PScreen_PauseMenu:163:in `pbStartPokemonMenu'

PScreen_PauseMenu:144:in `loop'

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Not working solution:
Replace
form = ($Trainer.formlastseen[nationalSpecies][1] || 0)

with this
if defined?($Trainer.formlastseen[nationalSpecies][1])
form = ($Trainer.formlastseen[nationalSpecies][1] || 0)
else
form = 0
end

Have you tried starting a new save file or deleting your save from C:\Users\USERNAME\Saved Games ?

double check that every pokemon has a dex number by using the debug menus edit pokedex feature. and '-------------------' that is between two pokemon means you're missing a pokemon which might be causing your error