According to the spec for javax.swing.plaf.metal.MetalMenuBarUI the method update(Graphics, JComponent) should throw NPE if any of the arguments are null.
However if we invoke update() with null Graphics argument then NPE is not thrown. You can reproduce it by following code:

import javax.swing.JComponent;
import javax.swing.plaf.metal.MetalMenuBarUI;

public class Test {
    public static void main(String[] args) {
       JComponent jComp = new JComponent(){};
       MetalMenuBarUI menuBarUI = new MetalMenuBarUI();
       menuBarUI.update(null, jComp);
       System.out.println("OK");
   }
}

It prints OK instead of throwing NPE.

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