ADDITIONAL SYSTEM INFORMATION :
Windows 10
Java 13.0.2 (Azul jdk)
JavaFX 13.0.2
JMetro 11.6.7
Gradel: 6.2.1

A DESCRIPTION OF THE PROBLEM :
I seems like when switching between skins, the dispose method is not cleaning up listeners properly. I found this while working with JMetro (https://github.com/JFXtras/jfxtras-styles/tree/master/src/jmetro). I started my application using the default theme, and then when I tried to switch to JMetro, I got an error. I reported this to the developer of JMetro (https://github.com/JFXtras/jfxtras-styles/issues/144), who believes that it is actually a JavaFX issue.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Start the Application without using JMetro
2. Switch to using JMetro

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
UI should change to JMetro theme, with no errors.
ACTUAL -
UI does look like it changes to JMetro, but the following error occurs:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at javafx.controls/javafx.scene.control.skin.TextFieldSkin.updateSelection(TextFieldSkin.java:740)
at javafx.controls/javafx.scene.control.skin.TextFieldSkin.lambda$new$3(TextFieldSkin.java:233)
at javafx.base/com.sun.javafx.binding.ExpressionHelper$SingleInvalidation.fireValueChangedEvent(ExpressionHelper.java:136)
at javafx.base/com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.base/javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(ObjectPropertyBase.java:106)
at javafx.base/javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:113)
at javafx.base/javafx.beans.property.ObjectPropertyBase$Listener.invalidated(ObjectPropertyBase.java:234)
at javafx.base/com.sun.javafx.binding.ExpressionHelper$SingleInvalidation.fireValueChangedEvent(ExpressionHelper.java:136)
at javafx.base/com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.base/javafx.beans.binding.ObjectBinding.invalidate(ObjectBinding.java:170)
at javafx.graphics/javafx.scene.text.Text.doGeomChanged(Text.java:847)
at javafx.graphics/javafx.scene.text.Text$1.doGeomChanged(Text.java:159)
at javafx.graphics/com.sun.javafx.scene.shape.TextHelper.geomChangedImpl(TextHelper.java:106)
at javafx.graphics/com.sun.javafx.scene.NodeHelper.geomChanged(NodeHelper.java:137)
at javafx.graphics/javafx.scene.text.Text$9.invalidated(Text.java:831)
at javafx.base/javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
at javafx.base/javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:147)
at javafx.graphics/javafx.css.StyleableObjectProperty.set(StyleableObjectProperty.java:82)
at javafx.graphics/javafx.css.StyleableObjectProperty.applyStyle(StyleableObjectProperty.java:68)
at javafx.graphics/javafx.scene.CssStyleHelper.transitionToState(CssStyleHelper.java:787)
at javafx.graphics/javafx.scene.Node.doProcessCSS(Node.java:9660)
at javafx.graphics/javafx.scene.Node$1.doProcessCSS(Node.java:472)
at javafx.graphics/com.sun.javafx.scene.NodeHelper.processCSSImpl(NodeHelper.java:192)
at javafx.graphics/com.sun.javafx.scene.NodeHelper.processCSS(NodeHelper.java:145)
at javafx.graphics/javafx.scene.Parent.doProcessCSS(Parent.java:1399)
at javafx.graphics/javafx.scene.Parent$1.doProcessCSS(Parent.java:125)
at javafx.graphics/com.sun.javafx.scene.ParentHelper.processCSSImpl(ParentHelper.java:98)
at javafx.graphics/com.sun.javafx.scene.NodeHelper.processCSS(NodeHelper.java:145)
at javafx.graphics/javafx.scene.Parent.doProcessCSS(Parent.java:1399)
at javafx.graphics/javafx.scene.Parent$1.doProcessCSS(Parent.java:125)
at javafx.graphics/com.sun.javafx.scene.ParentHelper.processCSSImpl(ParentHelper.java:98)
at javafx.controls/com.sun.javafx.scene.control.ControlHelper.superProcessCSSImpl(ControlHelper.java:63)
at javafx.controls/com.sun.javafx.scene.control.ControlHelper.superProcessCSS(ControlHelper.java:55)
at javafx.controls/javafx.scene.control.Control.doProcessCSS(Control.java:886)
at javafx.controls/javafx.scene.control.Control$1.doProcessCSS(Control.java:89)
at javafx.controls/com.sun.javafx.scene.control.ControlHelper.processCSSImpl(ControlHelper.java:67)
at javafx.graphics/com.sun.javafx.scene.NodeHelper.processCSS(NodeHelper.java:145)
at javafx.graphics/javafx.scene.Parent.doProcessCSS(Parent.java:1399)
at javafx.graphics/javafx.scene.Parent$1.doProcessCSS(Parent.java:125)
at javafx.graphics/com.sun.javafx.scene.ParentHelper.processCSSImpl(ParentHelper.java:98)
at javafx.graphics/com.sun.javafx.scene.NodeHelper.processCSS(NodeHelper.java:145)
at javafx.graphics/javafx.scene.Node.processCSS(Node.java:9542)
at javafx.graphics/javafx.scene.Scene.doCSSPass(Scene.java:569)
at javafx.graphics/javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2469)
at javafx.graphics/com.sun.javafx.tk.Toolkit.lambda$runPulse$2(Toolkit.java:412)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at javafx.graphics/com.sun.javafx.tk.Toolkit.runPulse(Toolkit.java:411)
at javafx.graphics/com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:438)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:563)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:543)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulseFromQueue(QuantumToolkit.java:536)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.lambda$runToolkit$11(QuantumToolkit.java:342)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
at java.base/java.lang.Thread.run(Thread.java:830)

---------- BEGIN SOURCE ----------
package ca.ewert.jmetrotest;

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.*;
import javafx.stage.Stage;
import jfxtras.styles.jmetro.*;

public class JMetroTestApplication extends Application {

  public static void main(String[] args) {
    launch(args);
  }

  private Scene scene;

  @Override
  public void start(Stage primaryStage) {
    String javaVersion = System.getProperty("java.version");
    String javafxVersion = System.getProperty("javafx.version");
    Label label = new Label("Hello, JavaFX " + javafxVersion + ", running on Java " + javaVersion + ".");
    TextField textField = new TextField();
    CheckBox checkBox = new CheckBox("Hello JavaFx");
    Button button = new Button("Switch to JMetro");
    button.setOnAction(event -> {
      this.switchToJMetro();
    });

    VBox vBox = new VBox();
    vBox.setPadding(new Insets(10.0));
    vBox.setSpacing(5.0);
    vBox.getChildren().addAll(label, textField, checkBox, button);
    this.scene = new Scene(vBox, 800, 600);
    //this.switchToJMetro();
    primaryStage.setScene(scene);
    primaryStage.setTitle("JMetro Test");
    primaryStage.show();
  }

  private void switchToJMetro() {
    System.out.println("Switching to JMetro");
    JMetro jMetro = new JMetro(Style.LIGHT);
    jMetro.setScene(this.scene);
  }
}
---------- END SOURCE ----------

FREQUENCY : always