XoColor: don't choke if passed the wrong type (SL#1408)

Color data is stored in the Journal as a JSON dump, so we can get back
arbitrary types. XoColor already checks the color string for validity and
handles invalid strings gracefully, so it makes sense to enhance this to
invalid types as well.

Acked-by: Simon Schampijer <simon@schampijer.de>

src/sugar/graphics/xocolor.py

@@ -209,6 +209,10 @@ colors = [
 
 
 def _parse_string(color_string):
+    if not isinstance(color_string, (str, unicode)):
+        logging.error('Invalid color string: %r', color_string)
+        return None
+
     if color_string == 'white':
         return ['#ffffff', '#414141']
     elif color_string == 'insensitive':