Before, EditorViewport would start accessing arrays on keypresses (specifically
the arrow keys) even when there were no editors.  This adds an extra check to
prevent this.

Sometimes initialize() returns no error string, but will not find an audio
device, leaving aIOd == null.  This checks for that, and aborts the app with an
informational dialog window if so.

If there was a problem initializing the AudioDeviceManager, we show an
AlertWindow with Retry/Quit options.  Unfortunately, this doesn't fix the bug I
was initially trying to fix, because sometimes initialize() returns OK while
setting the AudioDeviceManager to null.