The code was relying on the layer cache through `groups_for_write`/`layers_for_write`.
The `group->runtime->layer_group_cache_` stores all the child groups instead
of groups on first level. This leads to crash when recursively deleting
group i.e. group at third level has been deleted but `layer_group_cache_`
of first level group will still point to address of freed memory.
The fix removes the use of the cache and iterates over the direct
children in `group.children` and deletes them recursively.
Pull Request: https://projects.blender.org/blender/blender/pulls/130082
9a59372f07