5d8e29f84e
Caused by 9f90594db7
`func_argN_copy_fn` is assigned when `UI_but/block_funcN_set` is used.
In case of `ui_but_new`, it is uninitialized. Use block's callback
for but if `block/but->func_argN` exists.
Pull Request: https://projects.blender.org/blender/blender/pulls/124388