Optimizely Forms show a blank page when creating new element

How to recreate the bug:
1) Create a new Alloy project with CMS 12.
2) Install EPiServer.Forms nuget package. I used version 5.2.0 in my case.

3) Navigate to a content area and create a new Forms container block. Do not publish the block.

4) Create a new Forms element (like text element) on the container block.

5) The result is a blank page. You cannot see the blocks properties or create the block.

It works fine if you publish the Forms container before adding any Form elements.
It took me a while to figure out what was wrong and I suspect our editors will have the same problem.