When a style class is applied to a node using NodeBuilder#styleClass(), instantiation of the node fails, because NodeBuilder calls getStyleClass().setAll() rather than addAll() to apply the custom style classes. The following example demonstrates the problem:

NodeBuilderTest.java:

package test;

import java.util.Arrays;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.LabelBuilder;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class NodeBuilderTest extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {
        VBox root = new VBox();
        root.getChildren().add(LabelBuilder.create().text("Hello World").styleClass(Arrays.asList(new String[] {"redLabel"})).build());
        Scene scene = new Scene(root, 640, 480);
        scene.getStylesheets().add("test/node_builder_test.css");
        primaryStage.setScene(scene);
        primaryStage.show();
    }
    
    public static void main(String[] args) {
        launch();
    }
}

node_builder_test.css:

.redLabel {
    -fx-text-fill: #ff0000;
}