[chart power query] An error message occurs when user saves a chart using chart power query

[andrey-zayats]

How to reproduce the bug

• Go to SQL lab
• Run a query
• Hit the "Create chart" button
• Hit the "Save" button, and save the chart
• See the error

Expected results

All controls remain filled in, no errors appear after saving a chart using chart power query

Actual results

User sees an unexpected error after saving a chart using chart power query, see the video below

empty.query.mp4

Screenshots

Environment

latest master

• browser type and version:
• superset version: superset version
• python version: python --version
• node.js version: node -v
• any feature flags active:

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

Add any other context about the problem here.