85ce2a34e3
This was caused by ab84f9eaf0.
The issue is that setting the flag in the `TreeNode`
constructor means that the root group will always
hide masks when its created. This effectively disables
masks in the entire object.
Instead, leave the default flag to be `0` (masks enabled)
and explicitly set the `HIDE_MASKS` flag when
creating a new layer/layer group.
Pull Request: https://projects.blender.org/blender/blender/pulls/120218