Introspect proxy's parent only it is a model

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/navigation/classes.py

@@ -378,6 +378,7 @@ class Menu(object):
                         if type(resolved_navigation_object) == bound_source:
                             # Check to see if object is a proxy model. If it is, add its parent model
                             # menu links too.
+                            if hasattr(resolved_navigation_object, '_meta'):
                                 parent_model = resolved_navigation_object._meta.proxy_for_model
                                 if parent_model:
                                     parent_instance = parent_model.objects.filter(pk=resolved_navigation_object.pk)