Warnings about invalid checkout URL does not show if checkout page is deleted

### Reproduction Steps

1. Ensure a checkout page is selected under LifterLMS > Settings > Checkout
2. Go to Pages, and permanently delete the page
3. Navigate to Courses, and edit a course with access plans

### Expected Behavior

A warning appears that the checkout page is incorrectly configured

### Actual Behavior

No warning

https://share.zight.com/8Luz4lWK