This is related to an old post: https://world.episerver.com/forum/developer-forum/-Episerver-75-CMS/Thread-Container/2019/1/cannot-read-property-contentdata-of-null----publish/
The issue was that the Publish menu was not responding due to a console error "Cannot read property 'ContentData' of null'. This was caused by a bug in Episerver.Marketing.Testing, and apparently fixed in v 2.5.5.
Just a heads up: I experienced the same problem running Episerver.CMS 11.13 and Episerver.Marketing.Testing 2.5.10.
The solution: I had to remove all temporary files from the browser.