add a check to confirm we have a level object.

[ipokkel]

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• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Added a check to confirm that we have a level object to ensure the level name attribute is available when we create an approval status message.

Closes Issue: #187

How to test the changes in this Pull Request:

1. Enable WP Debug and debug display.
2. Check out for a level that requires approval.
3. As admin, delete that level.
4. Return to the Membership Account page for the user that needed approval.
5. Confirm that the message reported in issue #187 is no longer displayed.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.