A DESCRIPTION OF THE PROBLEM :
In case I create a JSpinner add a PropertyChangeListener and then call setFont(null) it results in a NullPointerException. It looks like the bug was introduced in Java 9 with the fix:
https://bugs.openjdk.java.net/browse/JDK-5036022
Instead of passing the font it has been wrapped in a FontUIResource first, but the FontUIResource does not allow null font. 
 tf.setFont(new FontUIResource(spinner.getFont())); 
The line should be:
 tf.setFont(spinner. getFont() == null ? null : new FontUIResource(spinner.getFont()));

REGRESSION : Last worked in version 8u172

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create a JSpinner
Add a PropertyChangeListener
Call setFont(null)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
NullPointerException should not be thrown
ACTUAL -
Exception in thread "main" java.lang.NullPointerException
	at java.desktop/java.awt.Font.<init>(Font.java:712)
	at java.desktop/javax.swing.plaf.FontUIResource.<init>(FontUIResource.java:68)
	at java.desktop/javax.swing.plaf.basic.BasicSpinnerUI$Handler.propertyChange(BasicSpinnerUI.java:1020)
	at java.desktop/java.beans.PropertyChangeSupport.fire(PropertyChangeSupport.java:341)
	at java.desktop/java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:333)
	at java.desktop/java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:266)
	at java.desktop/java.awt.Component.firePropertyChange(Component.java:8728)
	at java.desktop/java.awt.Component.setFont(Component.java:1946)
	at java.desktop/java.awt.Container.setFont(Container.java:1777)
	at java.desktop/javax.swing.JComponent.setFont(JComponent.java:2769)
	at JSpinnerTest.main(JSpinnerTest.java:14)

---------- BEGIN SOURCE ----------
import javax.swing.JSpinner;

public class JSpinnerTest {
	public static void main(String[] args) {
		JSpinner spinner = new JSpinner();
		spinner.addPropertyChangeListener(evt -> {});
		spinner.setFont(null);
	}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
We could avoid this bug by not calling the setFont with null, but unfortunately, it is in an external library, so we cannot change.

FREQUENCY : always