When setting the content on a scrollpane, content.getParent() returns null as long as the stage is not visible - see test below.

public class ScrollPaneContentParentTest extends Application
{
  private Pane scrollContent;
  
  public static void main(String[] args)
  {
    Application.launch(args);
  }

  @Override
  public void start(Stage stage)
  {
    Parent root = createContent();
    System.err.println("Parent (shouldn't be null): " + scrollContent.getParent());
    Scene scene = new Scene(root, 500, 500);
    stage.setScene(scene);
    stage.show();
    System.err.println("Parent: " + scrollContent.getParent());
  }
  
  private Parent createContent()
  {
    ScrollPane scroller = new ScrollPane();
    scrollContent = new FlowPane(10,10);
    scrollContent.setPrefSize(500, 2000);
    Button button = new Button("MyButton");
    scrollContent.getChildren().add(button);
    scroller.setContent(scrollContent);
    return scroller;
  }
}