"undefined method `unlock' for nil:NilClass" error on map transfer

Exception: NoMethodError
Message: undefined method `unlock' for nil:NilClass
Interpreter:512:in `command_end'
Interpreter:295:in `execute_command'
Interpreter:193:in `update'
Interpreter:106:in `loop'
Interpreter:198:in `update'
Scene_Map:103:in `update'
Scene_Map:101:in `loop'
Scene_Map:114:in `update'
Scene_Map:68:in `main'
Scene_Map:65:in `loop'

I don't understand what's happened. I've checked the transfer player events and both follow the examples in pre-con maps.

Added any new script?

Had the same exact problem: this should help :)
https://www.pokecommunity.com/showthread.php?t=351258