[orca] Be sure we have a default screen before trying to connect callbacks for it

[Joanmarie Diggs <joanied src gnome org>]

commit 418fc296d79ef1be256eb5b2301b877af7054c53
Author: Joanmarie Diggs <jdiggs igalia com>
Date:   Sat Jan 21 16:57:01 2017 -0500

    Be sure we have a default screen before trying to connect callbacks for it

 src/orca/mouse_review.py |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/src/orca/mouse_review.py b/src/orca/mouse_review.py
index bea0249..8fa4480 100644
--- a/src/orca/mouse_review.py
+++ b/src/orca/mouse_review.py
@@ -205,8 +205,9 @@ class MouseReviewer:
 
         _eventManager.registerModuleListeners(self._get_listeners())
         screen = Wnck.Screen.get_default()
-        i = screen.connect("window-stacking-changed", self._on_stacking_changed)
-        self._handlerIds[i] = screen
+        if screen:
+            i = screen.connect("window-stacking-changed", self._on_stacking_changed)
+            self._handlerIds[i] = screen
 
     def deactivate(self):
         """Deactivates mouse review."""