[Explore] Single metric selector is inoperable if the referenced metric no longer exists

If a metric is removed from a datasource the single metric widget is inoperable if saved slice or URL references the previously defined metric. Note this isn’t a problem for visualization types which support multiple metrics.

Expected results

The metric widget to render correctly.

Actual results

The metric widget in inoperable (see attached screenshot).

Screenshots

How to reproduce the bug

1. Using the example data visit the “Number of Girls” chart.
2. Edit the datasource by deleting the sum_num metric and then clicking SAVE.
3. Reload the page.
4. Verify the METRIC widget is inoperable (see screenshot).

Environment

(please complete the following information):

• superset version: master
• python version: 3.7
• node.js version: 14

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.

[Bonifacio2]

I’ll take a look at this over the weekend.

[rusackas]

@kgabryje said he’d be willing to do a little investigation here, so I’m (at least temporarily) sticking his name on it 😃