Huh, just installed HTOP from the repos for 11.1 and see two windows come up when activated from xwbar icon - one empty window and one window with htop in it.
Closing the empty window closes both itself AND the window with htop in it.
When htop is invoked from within an already existing aterm, htop acts normally and no extra empty window appears.
This is on a recent CorePlus 11.1 where I pulled down HTOP.
Not a big deal.