[#2731] [GTKUI] Fix potential AttributeError in is_on_active_workspace

* Without being able to replicate adding the forced updated is the likely fix for 'win'
being None but also add test in case it's not...

deluge/ui/gtkui/mainwindow.py

@@ -339,11 +339,13 @@ class MainWindow(component.Component):
             bool: True if on active workspace (or wnck module not available), otherwise False.
 
         """
-        if not wnck:
-            return True
+        if wnck:
+            self.screen.force_update()
             win = wnck.window_get(self.window.window.xid)
+            if win:
                 active_wksp = win.get_screen().get_active_workspace()
                 if active_wksp:
                     return win.is_on_workspace(active_wksp)
                 else:
                     return False
+        return True