When undoing a convert to container. there is a nasty bug that deletes everything that was in the group and it is impossible to undo / redo. Please fix this. Or make it not possible to undo a contianer at the very least so not everything is lost.
Edit* For a window the best option would be to close and revert. But if this happens after you have done a bunch of work all of that would be lost, or on a template that is set for auto commit.