Click on a folder in the (left) folder structure panel. It will highlight, and its contents will appear in the right panel. That's OK.
Select a file on the right and press 'delete'. It will correctly try to delete that file.
Select a folder in the left panel. It will highlight, its contents will appear right, and no file at all is selected (nor highlighted).
Press 'delete'. You'd expect that the left folder would delete, but instead it tries to delete a file on the right...