Verify that item.by is not None before calling render_list_of_roles

wcs/admin/workflows.ptl

@@ -111,7 +111,7 @@ def graphviz(workflow, url_prefix='', select=None, svg=True,
             url = 'status/%s/items/%s/' % (i, item.id)
             if hasattr(item, 'label'):
                 label = item.label
-                if hasattr(item, 'by'):
+                if getattr(item, 'by', None):
                     roles = render_list_of_roles(item.by)
                     label += ' %s %s' % (_('by'), roles)
             else: