getDisplayMode() returns null for the default graphics device on linux-arm (and Linux VM).
According to Oracle docs the returned value can not be null.

The issue can be seen with this code:

GraphicsDevice gd = GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice();
DisplayMode dm = gd.getDisplayMode();
if(dm == null) {
       System.err.println("DisplayMode is null");
 } else {
      System.err.println("DisplayMode=" + dm.toString());
 }

We do not observe the issue on linux-i586.

We have several functional tests failing because of this issue.