The spec for javax.swing.plaf.nimbus.AbstractRegionPainter.PaintContext says:

         * @param insets The stretching insets. May be null. If null, then assumed to be 0, 0, 0, 0.

In fact NPE is thrown. Please see the following code sample:
------------------------------------------------------------
import javax.swing.*;
import java.awt.*;

public class Test {

    public static void main(String[] args) {

        class MyARP extends javax.swing.plaf.nimbus.AbstractRegionPainter {
            protected PaintContext getPaintContext() {
                return null;
            }
            protected void doPaint(Graphics2D g, JComponent c,
                                   int width, int height,
                                   Object[] extendedCacheKeys) {
            }
            void createPaintContext() {
                new PaintContext(null, new Dimension(), true);
            }
        }
        new MyARP().createPaintContext();
    }
}
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The result will be:

Exception in thread "main" java.lang.NullPointerException
at javax.swing.plaf.nimbus.AbstractRegionPainter$PaintContext.<init>(AbstractRegionPainter.java:541)
at javax.swing.plaf.nimbus.AbstractRegionPainter$PaintContext.<init>(AbstractRegionPainter.java:506)
at Test$1MyARP.createPaintContext(Test.java:17)
at Test.main(Test.java:20)