Fix the PHP warning and hide the form until a list is selected in the settings.

[iamdharmesh]

Description of the Change

This PR updates the error-handling code to prevent PHP warnings during form submission. Additionally, it includes the following changes:

• Hide additional settings until a list is selected in the settings.
• Clear the selected list option upon logout.
• Hide the signup form on the front end if no list is selected in the settings.

Closes #53

How to test the Change

1. Log out if you have a Mailchimp account connected.
2. Log in to your Mailchimp account and connect it.
3. Verify that all settings remain hidden until a list is selected and saved from the dropdown.
4. Verify on the front end that no form is visible until a list is selected and saved from the dropdown in the settings.
5. Select any list from the dropdown and save the settings.
6. Verify that the form on the front end appears and functions without any issues.

Changelog Entry

Changed - Make the signup form visible only when a list is selected in the settings.
Fixed - Ensure that the signup form submission works correctly without any PHP errors or warnings.

Credits

Props @dkotter @iamdharmesh

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.