Move close handling to windowClosed event

This callback is called more reliably than windowClosing when killing
window from outside.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

runelite-client/src/main/java/net/runelite/client/util/SwingUtil.java

@@ -216,10 +216,16 @@ public class SwingUtil
 
 				if (result == JOptionPane.OK_OPTION)
 				{
-					callback.run();
-					System.exit(0);
+					frame.dispose();
 				}
 			}
+
+			@Override
+			public void windowClosed(WindowEvent event)
+			{
+				callback.run();
+				System.exit(0);
+			}
 		});
 	}