Upgrade to v4.0.6 give xDebug issue

@LukeTowers
On a working project, if I update to Winter.Debugbar v4.0.6, I ran into a xDebug infinite loop warning when trying to put a model in cache.

How to fix the issue:

• Revert to version 4.0.5
  or
• Comment this line in Plugin.php https://github.com/wintercms/wn-debugbar-plugin/blob/9a709a8821929338648a132ef20dc4bb9a249cb0/Plugin.php#L58
  or
• Replace config winter.debugbar.options.mail with :

        'mail' => [
            // 'full_log' => false
            'timeline' => env('DEBUGBAR_OPTIONS_MAIL_TIMELINE', true),  // Add mails to the timeline
            'show_body' => env('DEBUGBAR_OPTIONS_MAIL_SHOW_BODY', false),
        ],

In this latest case, what I don't understand is having to set the value of options.mail.timeline to true.
If I set it to false, I get the same error.

Start with Plugin.php at line 58 and compare the v4.0.5 and v4.0.6 behavior around the winter.debugbar.options.mail configuration. Reproduce the xDebug infinite-loop warning when caching a model, then verify that the fix removes the warning without requiring the mail timeline option to be enabled.