res7art The Akismet extension is causing this issue.
Due to the Akismet settings bypassing administrators, while administrators cannot create discussions with empty content, members who are subject to Akismet checks can attempt to, which triggers the problem.
The akismet/src/Listener/ValidatePost.php file, when processing the incoming request, does not expect the content field to be empty (null) and throws a TypeError.
Interestingly, after this error occurs and you click 'share' again, the content is sent as \n instead of being truly empty, and Akismet approves it.
This is an interesting conflict/interaction, and I don't expect it to be fixed quickly. I recommend disabling this extension for now.
@SychO @luceos