Unexpected error when creating lessons without parent course.

### Reproduction Steps

1. Start with a fresh install of WordPress and LifterLMS.
2. Go to **WordPress Dashboard > Courses > Lessons** and create a new lesson by clicking on the **Add Lesson** button.
3. Click on the **LifterLMS** button in [this screenshot](https://share.zight.com/L1uykXGZ).
4. Click on the **Settings** button in [this screenshot](https://share.zight.com/yAuBL1PN).
5. You'll get a popup saying "The editor has encountered an unexpected error.", as you can see in [this screenshot.](https://share.zight.com/2Nu9rmJ0)

### Expected Behavior
Should not get the [error in this screenshot](https://share.zight.com/2Nu9rmJ0).

### Actual Behavior
The user is getting the [error in this screenshot](https://share.zight.com/2Nu9rmJ0).

This issue has be recreated:
+ [x] Locally
+ [ ] On a staging site
+ [x] On a production website
+ [x] With only LifterLMS and a default theme