desktop: don't show dialogs when headless

jme3-desktop/src/main/java/com/jme3/system/JmeDesktopSystem.java

@@ -43,6 +43,7 @@ import com.jme3.system.JmeContext.Type;
 import com.jme3.util.Screenshots;
 import java.awt.EventQueue;
 import java.awt.Graphics2D;
+import java.awt.GraphicsEnvironment;
 import java.awt.RenderingHints;
 import java.awt.geom.AffineTransform;
 import java.awt.image.AffineTransformOp;
@@ -116,12 +117,16 @@ public class JmeDesktopSystem extends JmeSystemDelegate {
 
     @Override
     public void showErrorDialog(String message) {
+        if (!GraphicsEnvironment.isHeadless()) {
             final String msg = message;
             EventQueue.invokeLater(new Runnable() {
                 public void run() {
                     ErrorDialog.showDialog(msg);
                 }
             });
+        } else {
+            System.err.println("[JME ERROR] " + message);
+        }
     }
 
     @Override
@@ -129,6 +134,9 @@ public class JmeDesktopSystem extends JmeSystemDelegate {
         if (SwingUtilities.isEventDispatchThread()) {
             throw new IllegalStateException("Cannot run from EDT");
         }
+        if (GraphicsEnvironment.isHeadless()) {
+            throw new IllegalStateException("Cannot show dialog in headless environment");
+        }
 
         final AppSettings settings = new AppSettings(false);
         settings.copyFrom(sourceSettings);