That's strange, ESC works fine for me.
In the viewer/editor, when you open the File menu, doesn't it say 'Close ESC' near the bottom?
To (hopefully) fix this, go to View | Customize | Keyboard in the viewer's menu. Then look for Close in the File category. On my system, ESC, Ctrl-F4 and Alt-F4 are all three assigned to the Close command.