Can not move node groups with the form
If I try to move a group from a category to an other with the "Group container" field and save, that leads to:
The form contains some errors, please correct them.
There are no modifications to save
If I try to drag'n'drop the group from a category to another, it works.
Merge pull request #816 from fanf/bug_4571/can_not_move_node_groups_with_the_form
Fixes #4571: Can not move node groups with the form