small fix for show/hide ChRtSpecification command

plugins/org.polarsys.chess.commands/src/org/polarsys/chess/commands/ShowRTInformationCommand.java

@@ -122,7 +122,10 @@ public class ShowRTInformationCommand extends AbstractHandler{
 			if (temp instanceof BasicCompartmentImpl){
 				Iterator iter2 = ((BasicCompartmentImpl) temp).getPersistedChildren().iterator();
 				while (iter2.hasNext()){
-					showHideRT((Shape)iter2.next(), visibility);
+					Object obj = iter2.next();
+					if(obj instanceof Shape){
+						showHideRT((Shape)obj, visibility);
+					}
 				}
 			}
 		}