Fix logic for Popup closing when parent is focused

The member popped_up is used to avoid closing a Popup before it had a
chance to be focused. It wasn't reset properly when the popup is hidden,
causing the Popup to close right after showing in some random cases
(spotted on X11, might not happen on Windows).

scene/gui/popup.cpp

@@ -72,6 +72,7 @@ void Popup::_notification(int p_what) {
 			} else {
 				_deinitialize_visible_parents();
 				emit_signal(SNAME("popup_hide"));
+				popped_up = false;
 			}
 
 		} break;