palettemenuwidget: Ensure the widget is realized before popping it up, SL #4388

Otherwise the internal state is messed up, GTK+ grabs may be held on
unrealized widgets.

Signed-off-by: Carlos Garnacho <carlos@lanedo.com>
Acked-by: Simon Schampijer <simon@laptop.org>

src/sugar3/graphics/palettewindow.py

@@ -148,6 +148,7 @@ class _PaletteMenuWidget(Gtk.Menu):
 
         self._invoker = invoker
         self._find_all_menus(self)
+        self.realize()
         for menu in self._menus:
             if self._invoker:
                 menu.connect('motion-notify-event', self._motion_notify_cb)