The folder clearly isn’t empty otherwise avast wouldn’t be running in any form and Yes I do mean in the blank area as in:

click the mouse left button inside the blank window.
That is essentially to have what is called focus (e.g. that window is the active one), so that the Ctrl and + keys effect the blank window and not another one.

If you have tried this and it didn’t work I’m as much at a loss as you because I spent ages googling to try and find solutions. As I said previously I’ve never seen this before and there is definitely something screwy going on in your system.