Code page when clicking "Post Reply" in forum

I clicked the "Post Reply" Button by accident, while I hadn't entered a reply.

I got moved to a white page with code: https://imgur.com/jFE9G3r (I tried to enter the image, but when I clicked "Save", after entering the URL, I got the message: The entered data is invalid). 

I assume, there is supposed to be an error message like "Post required".

I have tried it in multiple threads and it seems to be the same everywhere. So, a general bug and not a thread-specific one.