macOS: Fixed NPE if using JMenuBar in JInternalFrame and macOS screen menu bar is enabled (issue #90)

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -38,6 +38,7 @@ import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.logging.Level;
 import java.util.logging.Logger;
+import javax.swing.BorderFactory;
 import javax.swing.Icon;
 import javax.swing.ImageIcon;
 import javax.swing.JComponent;
@@ -306,9 +307,13 @@ public abstract class FlatLaf
 			UIDefaultsLoader.loadDefaultsFromProperties( getClass(), addons, getAdditionalDefaults(), isDark(), defaults );
 
 		// use Aqua MenuBarUI if Mac screen menubar is enabled
-		if( SystemInfo.IS_MAC && Boolean.getBoolean( "apple.laf.useScreenMenuBar" ) )
+		if( SystemInfo.IS_MAC && Boolean.getBoolean( "apple.laf.useScreenMenuBar" ) ) {
 			defaults.put( "MenuBarUI", "com.apple.laf.AquaMenuBarUI" );
 
+			// add defaults necessary for AquaMenuBarUI
+			defaults.put( "MenuBar.backgroundPainter", BorderFactory.createEmptyBorder() );
+		}
+
 		// initialize text antialiasing
 		putAATextInfo( defaults );
 

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatMenuBarUI.java

@@ -49,6 +49,11 @@ public class FlatMenuBarUI
 		return new FlatMenuBarUI();
 	}
 
+	/*
+	 * WARNING: This class is not used on macOS.
+	 *          Do not add any functionality here.
+	 */
+
 	@Override
 	protected void installKeyboardActions() {
 		super.installKeyboardActions();