Show an error message and return early if no list is chosen

[dkotter]

Description of the Change

As reported in #37 (comment), if someone tries to update a list without actually selecting a list, a fatal error occurs.

This PR fixes that by ensuring we have a valid value first and if not, return early and show an error message.

Partially closes #37

How to test the Change

1. Install and activate the plugin
2. Go to the settings page and enter a valid API key
3. Change the selected list to be the default, no list selected value
4. Click the update button and ensure an error message shows but no fatals occur

Changelog Entry

Fixed - Properly handle the scenario where a non-valid list is updated.

Credits

Props @dkotter, @qasumitbagthariya

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[nateconley]

@dkotter I noticed a couple of things and pushed up new commits:

1. The form action was never updated to the new top level menu page 8c0152f
2. There were persistent error messages when the list form was not being posted 3619ac5