iFrame doesn't work, and it seems to be introduced in #5753
Bug replication steps:
1. Change visualization type to iFrame in any Explore view
2. A exception is thrown
```
Traceback (most recent call last):
File "/home/eric/Documents/github/incubator-superset/superset/views/core.py",
line 1134, in generate_json
payload = viz_obj.get_payload()
File "/home/eric/Documents/github/incubator-superset/superset/viz.py", line
375, in get_payload
payload['data'] = self.get_data(df)
File "/home/eric/Documents/github/incubator-superset/superset/viz.py", line
489, in get_data
return self.get_df().to_dict(orient='records')
AttributeError: 'NoneType' object has no attribute 'to_dict'
```
[ Full content available at:
https://github.com/apache/incubator-superset/pull/5965 ]
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