Added check against the validity of ViewerBase to make sure that a null pointer isn't dereferenced.
Added check against the validity of ViewerBase to make sure that a null pointer isn't dereferenced.


XiaoCC/osg,omega-hub/osg,stmh/osg,XiaoCC/osg,stmh/osg,omega-hub/osg,ghub/osg,XiaoCC/osg,stmh/osg,koshman86/osg,XiaoCC/osg,koshman86/osg,ntoand/osg,ntoand/osg,XiaoCC/osg,artoolkit/osg,artoolkit/osg,ghub/osg,artoolkit/osg,koshman86/osg,stmh/osg,stmh/osg,ghub/osg,ntoand/osg,omega-hub/osg,omega-hub/osg,ghub/osg,ntoand/osg,koshman86/osg,artoolkit/osg