Since JDK12b20 the following:

        UIManager.setLookAndFeel(new SynthLookAndFeel());
        SynthButtonUI ui = new SynthButtonUI();
        JButton b1 = new JButton();
        JButton b2 = new JButton();
        b1.setUI(ui);
        b2.setUI(ui);

leads to NullPointerException

Exception in thread "main" java.lang.NullPointerException
at java.desktop/javax.swing.plaf.synth.SynthButtonUI.uninstallDefaults(SynthButtonUI.java:137)
at java.desktop/javax.swing.plaf.basic.BasicButtonUI.uninstallUI(BasicButtonUI.java:181)
at java.desktop/javax.swing.JComponent.uninstallUIAndProperties(JComponent.java:700)
at java.desktop/javax.swing.JComponent.setUI(JComponent.java:675)
at java.desktop/javax.swing.AbstractButton.setUI(AbstractButton.java:1753)