JRE-408 JBR-3515 fix NullPointerException in MetalRootPaneUI.installWindowListeners

(cherry picked from commit 584d554af529cff445b0f09bc2d57be55e138b7a)
(cherry picked from commit 6a42bb54bd)

src/java.desktop/share/classes/javax/swing/plaf/metal/MetalRootPaneUI.java

@@ -233,6 +233,7 @@ public class MetalRootPaneUI extends BasicRootPaneUI
      * @param parent The parent of the JRootPane
      */
     private void installWindowListeners(JRootPane root, Component parent) {
+        if (parent == null) return;
         if (parent instanceof Window) {
             window = (Window)parent;
         }