That’s what I would expect if you have a topic template set up on one of your categories. Without any set it should allow you to enter the composer text area before setting a category. It blocks it when using topic templates so people don’t accidentally bypass that template by typing in the composer first and then selecting a category.
I’ve just had a quick test run on my site, and with no default category or topic templates I can enter the composer area without being blocked until I select a category.