System File Chooser: on Linux when JavaFX is used in application, then always use Swing file chooser because system file dialog does work (PR #988)

with Java 8, some GLib related messages are logged to console and system file dialog is not shown
e.g.: `GLib-GObject-WARNING **: cannot register existing type 'GdkDisplayManager'`

with Java 17, the system file dialog is shown, but then JavaFX no longer works
with Java 21, the application quits/crashes immediately when trying to show system file dialog

also fixed Error Prone warning in `getFiltersForDialog()`

flatlaf-core/src/main/java/com/formdev/flatlaf/util/SystemFileChooser.java

@@ -22,9 +22,10 @@ import java.awt.SecondaryLoop;
 import java.awt.Toolkit;
 import java.awt.Window;
 import java.io.File;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Locale;
@@ -590,7 +591,7 @@ public class SystemFileChooser
 		if( filters.size() == 1 &&
 			filters.get( 0 ) == getAcceptAllFileFilter() &&
 			(fileFilter == getAcceptAllFileFilter() || fileFilter == null) )
-		  return Collections.emptyList();
+		  return new ArrayList<>();
 
 		// check whether current filter is already in list
 		if( (fileFilter != null && filters.contains( fileFilter )) || fileFilter == null )
@@ -1073,6 +1074,15 @@ public class SystemFileChooser
 	private static class LinuxFileChooserProvider
 		extends SystemFileChooserProvider
 	{
+		@Override
+		public File[] showDialog( Window owner, SystemFileChooser fc ) {
+			// fallback to Swing file chooser if JavaFX is initialized
+			if( isFXinitialized() )
+				return new SwingFileChooserProvider().showDialog( owner, fc );
+
+			return super.showDialog( owner, fc );
+		}
+
 		@Override
 		String[] showSystemDialog( Window owner, SystemFileChooser fc ) {
 			boolean open = (fc.getDialogType() == OPEN_DIALOG);
@@ -1175,6 +1185,41 @@ public class SystemFileChooser
 			return buf.toString();
 		}
 
+
+		private static Boolean fxinitialized;
+
+		boolean isFXinitialized() {
+			if( fxinitialized != null )
+				return fxinitialized;
+
+			// check whether JavaFX is available
+			try {
+				Class.forName( "javafx.application.Platform", false, getClass().getClassLoader() );
+			} catch( ClassNotFoundException ex ) {
+				// JavaFX is not available
+				fxinitialized = false;
+				return fxinitialized;
+			}
+
+			// check whether JavaFX is initialized
+			try {
+				Class<?> cls = Class.forName( "javafx.application.Platform" );
+				Method m = cls.getMethod( "runLater", Runnable.class );
+				m.invoke( null, (Runnable) () -> {} );
+				fxinitialized = true;
+				return fxinitialized;
+			} catch( InvocationTargetException ex ) {
+				if( ex.getCause() instanceof IllegalStateException )
+					return false; // JavaFX is available, but not (yet) initialized
+			} catch( Throwable ex ) {
+				// ignore
+			}
+
+			// other error --> assume that JavaFX is not initialized
+			fxinitialized = false;
+			return fxinitialized;
+		}
+
 		//---- class LinuxApproveContext ----
 
 		private static class LinuxApproveContext

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatSystemFileChooserTest.java

@@ -77,7 +77,7 @@ public class FlatSystemFileChooserTest
 	FlatSystemFileChooserTest() {
 		initComponents();
 
-		if( !NativeJFileChooser.FX_AVAILABLE ) {
+		if( SystemInfo.isLinux || !NativeJFileChooser.FX_AVAILABLE ) {
 			javafxOpenButton.setEnabled( false );
 			javafxSaveButton.setEnabled( false );
 		}