Continued intermittent NullPointerExceptions in GrayBoxPanel

An occasional NullPointerException from the GrayBoxPanel has been observed due to the inherent multithread-unsafety of this code. No stack trace was captured successfully at the time of observation, unfortunately. We would like to avoid having this non-essential code print any exceptions to the Java Console unless tracing is enabled. This is related to the previously fixed bug 6657116.