I only get these error. But it's different case, because the error happen after the onselect_level() get called again.
But no. Godot doesn't register the connect function.
Ah.. Wait.. I must put connect signal in the ready(). It cannot in the same function of the _onselect_level().
Yeah after a bit of looking up in the code. The problem seems to be what line being called first.
And it works.. But it will give the error in the first image.
Hahaha even though it's not related you point me to "signal handler not being called" made me realize my mistake though. Thanks.