Handle invalid /desktop/sugar/user/color

This is currently breaking the activity list unit test where
the profile has not been initialized. But, more in general,
it seems like we should not be crashing if our settings has
unexpected values.

src/sugar3/graphics/xocolor.py

@@ -239,7 +239,7 @@ class XoColor:
                           'fallback to default', color_string)
             client = GConf.Client.get_default()
             color_string = client.get_string('/desktop/sugar/user/color')
-            randomize = False
+            randomize = is_valid(color_string)
         else:
             randomize = False