I ran into this when I remote desktoped into my machine from home.
JCP saves the bounds of the cache viewer when it exits (or just when it is saved).
I had last run the cache viewer on a 2048x1600 screen in the office, and moved it to the right part of the screen.
When I later logged in from home with only a 1280x1024 screen, the cache viewer was placed at 1400,300. This was off the screen on my laptop, so I was froze. (had to use task manager to kill the JCP).

We should compare the position to the screen dimensions, and insure that at least some of the viewer is on the screen.