Fixed unit details view in Combat Ratio Breakdown Dialog to be cleared on close.

In the Combat Ratio Breakdown Dialog the middle section shows a detail graphic from any unit selected in the lists. If a unit was selected and details shown, then the dialog closed, on a subsequent opening of the dialog, this previously shown unit was still shown in the details area, even when nothing was selected on the lists. This is now fixed so that the unit details area always starts off as empty, corresponding to the nothing-selected status of the lists.