Caused by 03ae57dd8b
Custom preview operator uses `lib_id_preview_editing_poll`. To enable
this operator in UI, move the new condition to `lib_id_generate_preview_poll`
poll function so adding custom preview to unsupported type would
still be possible.
Co-authored-by: Julian Eisel
Pull Request: https://projects.blender.org/blender/blender/pulls/126874