Steps to reproduce the behavior

1. Go to https://galleryerror.wordpress.com/gallery-error/
2. Click on an image in the photo gallery.
3. Two JavaScript errors occurs: "Uncaught TypeError: can't access property "el", previous is undefined"
4. The "Title" of the media image is not displayed.

What I expected to happen

When a gallery image is clicked, the carousel lightbox should open and the the image title should be shown below the image. There should be no JavaScript error in the console.

What actually happened

When a gallery image is clicked two JavaScript errors appear in the console: "Uncaught TypeError: can't access property "el", previous is undefined." The carousel lightbox opens, but the image title is missing below the image.

Context

Browser / OS version

FireFox / OSX
Safari / OSX

Is this specific to the applied theme? Which one?

Does not seem to be specific to a theme.

Does this happen on simple or atomic sites or both?

Only simple sites.

Is there any console output or error text?

Yes: "Uncaught TypeError: can't access property "el", previous is undefined."

Level of impact (Does it block purchases? Does it affect more than just one site?)

Does NOT block purchase. It does affect more than one site. It has been witnessed on one site in the wild and reproduced on two separate test sites.

Reproducibility (Consistent, Intermittent) Leave empty for consistent.

Screenshot / Video: If applicable, add screenshots to help explain your problem.