fix Visualization.to_json() for loop (#15168)

bijoe/visualization/utils.py

@@ -62,7 +62,7 @@ class Visualization(object):
             'drilldown_x': self.drilldown_x and self.drilldown_x.name,
             'drilldown_y': self.drilldown_y and self.drilldown_y.name,
             'filters': self.filters,
-            'loop': self.loop,
+            'loop': self.loop and self.loop.name,
         }
 
     def copy(self):