If it occurs after you've quit, it's possible that it was a background task that was collecting data for one of the information screens. Sometimes the information gathering process involves calling out to third-party code which might not respond well if stopped in the middle. It isn't good that it caused an error, but it shouldn't harm your system.
If it happens every single time, even if you wait a while before quitting, there's probably something up in Tweak7 itself - perhaps we're making an assumption that isn't correct on your system. Again, there's nothing we really do on quitting that would affect your system, it would be more in the way of saving TweakVista's own settings.
If you have any more information about the problem, please send it over to email@example.com