Hi Geoff,
Yes, I am aware of this small problem. It only happens on Windows, and is innocuous. It happens sometimes when the SubframeSelector tool is made visible on a workspace when you load a project. The exception is being thrown by the web browser component used to implement SubframeSelector graphs. Just ignore it. Sometimes the graphs are shown incorrectly after this happens; in such case just force them to redraw (for example, by selecting a different graph in the Measurements Graph section) and everything will work normally.