Allow source columns without an attribute

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/navigation/classes.py

@@ -721,6 +721,8 @@ class SourceColumn(object):
             )
         elif self.func:
             result = self.func(context=context, **self.kwargs)
+        else:
+            result = context['object']
 
         if self.widget:
             widget_instance = self.widget()