We can take this big step to have a form before the list of forums for the user already insert content, something similar to the attached figure?
This is the type of UI most commonly used in the social networks. Users are already familiar with this, it would help a lot.
Instead of the "Select Forum" popup it could be a "next step" form/page for the user to choose the best place to post the topic he just wrote. This would help in addition to other things to reduce the sending of topics in the wrong forums.