Dragging an item in the menu editor as the first action results in an incorrect message

To reproduce on the archetype:

• Open a menu for editing
• Drag the "news" item to the right to make it a child item of "about"

Expected: no messages are shown

Actual: a tooltip message is shown for the label: "Label is required"

Also, it is possible to close the window at this point. Tested on Chrome and Safari. Note that the problem does not occur if you first select the "news" item.