ADDITIONAL SYSTEM INFORMATION :
Mac OS X 10.15. Java versions I tested with

openjdk version "12" 2019-03-19
OpenJDK Runtime Environment AdoptOpenJDK (build 12+33)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 12+33, mixed mode, sharing)

openjdk version "13" 2019-09-17
OpenJDK Runtime Environment AdoptOpenJDK (build 13+33)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 13+33, mixed mode, sharing)

java version "1.8.0_181"
Java(TM) SE Runtime Environment (build 1.8.0_181-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.181-b13, mixed mode)

A DESCRIPTION OF THE PROBLEM :
Whenever you have Apple Sidecar enabled then Swing applications crash with a NullPointerException. Once you disconnect the iPad and run the app again the crash disappears. This is present with the Java 8, 12 and 13 I tested with.

REGRESSION : Last worked in version 13

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Try to launch any Java application that uses the java.awt subsystem.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Application runs.
ACTUAL -
─$ java DemoFrame
Exception in thread "main" java.lang.NullPointerException
at java.awt.Window.init(Window.java:497)
at java.awt.Window.<init>(Window.java:537)
at java.awt.Frame.<init>(Frame.java:420)
at java.awt.Frame.<init>(Frame.java:385)
at javax.swing.JFrame.<init>(JFrame.java:189)
at DemoFrame.<init>(DemoFrame.java:3)
at DemoFrame.main(DemoFrame.java:5)

---------- BEGIN SOURCE ----------
Try to run a simple program that uses awt subsystem. For example

public class DemoFrame extends JFrame {
  public static void main(String[] args) {
    DemoFrame frame = new DemoFrame();
  }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Disconnect the sidecar'ed ipad and rerun your application for it to start up.

FREQUENCY : always