[Question] So how many problem threads have I had now?

[WinterKirby]

So I made a building where you can battle trainers, and you get items after battling them. I had already put three in there. However, when I made a fourth, I got this error as soon as I entered the building:

Exception: NoMethodError
Message: undefined method `*' for nil:NilClass
Game_Character_1:198:in `screen_z'
Sprite_Character:148:in `update_or'
PerspectiveTilemap:430:in `shadow_update'
Shadow:179:in `update'
Sprite_Character:37:in `perspectivetilemap_initialize'
PerspectiveTilemap:424:in `shadow_initialize'
Shadow:152:in `initialize'
Spriteset_Map:162:in `new'
Spriteset_Map:162:in `_animationSprite_initialize'
Spriteset_Map:161:in `each'

I tried taking him out and putting him in a different building, and I was able to go in and out fine, but was still unable to go into the original building. So it looks to me as if putting that trainer in caused something to go wrong, but I'm not sure about it, because it didn't happen with any other building.
Any idea why this happens? Because I have none.
It's like something's telling me I shouldn't be making games.