Fix ObjectChooser backwards compatibility

The last change brak compatibility with the use in activities
without set the filter_type parameter, because None is not
a allowed value in the dbus call.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>

src/sugar3/graphics/objectchooser.py

@@ -125,6 +125,10 @@ class ObjectChooser(object):
         else:
             what_filter = self._what_filter
 
+        if self._filter_type is None:
+            self._chooser_id = journal.ChooseObject(
+                self._parent_xid, what_filter)
+        else:
             self._chooser_id = journal.ChooseObjectWithFilter(
                 self._parent_xid, what_filter, self._filter_type)