Comment 1 for bug 1560329

Fortunately, this bug is not very noticeable because few of the blocks use required fields, and the failure mode only happens if you hit the "cancel" button, and even then, if you're not paying attention you may not notice the difference between the modal closing and the page reloading.

The most noticeable side effect is that it can lead to the creation of "empty" blocks on your page, like an External Feed block with no External Feed URL, or an External Video block with no video.