I know I know…I’m producing on a laptop but it’s a strong one. I’m doing F3 to open the mixer (pretty basic) and it’s not working. Also, how would I set up a command to close the midi editor?
For the first issue you may try to program a personal shortcut. This is faster than analyzing the error… 
Also, the mixer window could be behind another window.
For the second…a window is closed by CTRL - W, normally.
Hope this help.
Roger