Avoid warning by checking Level isn't null.

[MaximilianoRicoTabo]

All Submissions:

• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

Add a check for null to avoid the warning in the log.

How to test the changes in this Pull Request

Define constants below to get log file.
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
@ini_set('display_errors', 0);

See the log.

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.