Added extra check to prevent calling frame() when the window has been closed.

examples/osgkeyboardmouse/osgkeyboardmouse.cpp

@@ -391,11 +391,14 @@ int main( int argc, char **argv )
     {
         gw->checkEvents();
         
+        if (gw->isRealized() && !exitHandler->done())
+        {      
             viewer.frame();
 
             // Swap Buffers
             gw->swapBuffers();
         }
+    }
 
     return 0;
 }