Make sure we have a widget before attaching an invoker, SL #3460

The _ToolbarPalette does get passed the invoker on
initialisation. But we do create the PaletteWindowWidget
later. We do attach the invoker to the widget when calling
_setup_widget that is why it was still working without that
patch.

This patch prevents the traceback that we had because of not
having a widget at this point.

Signed-off-by: Simon Schampijer <simon@laptop.org>
Tested-by: Manuel Quiñones <manuq@laptop.org>

src/sugar3/graphics/palettewindow.py

@@ -498,7 +498,8 @@ class PaletteWindow(GObject.GObject):
             self._invoker_hids.remove(hid)
 
         self._invoker = invoker
-        self._widget.set_invoker(self._invoker)
+        if self._widget is not None:
+            self._widget.set_invoker(invoker)
         if invoker is not None:
             self._invoker_hids.append(self._invoker.connect(
                 'mouse-enter', self._invoker_mouse_enter_cb))