How to write Test cases for Javafx Controllers?

I have written this test class for javafx application.

class HelloApplicationTest extends GuiTest {

protected Parent getRootNode() {
    Parent parent = null;
    try {
        parent = FXMLLoader.load((getResource("hello-view.fxml")));
        return parent;
    catch (IOException ex) {

    return parent;
void add_button() {
    Button add_btn_1=find("#add_btn");
    FxAssert.verifyThat("#add_btn", LabeledMatchers.hasText("Add"));

I am getting error like this -> java.lang.NoSuchMethodError: 'java.util.Iterator javafx.stage.Window.impl_getWindows()'

<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="350.0" prefWidth="666.0" xmlns="" xmlns:fx="" fx:controller="com.example.fxml_db.HelloController">
   <Label layoutX="14.0" layoutY="14.0" prefHeight="39.0" prefWidth="295.0" text="Add and View Employee:">
        <Font name="System Bold" size="24.0" />
  <Label layoutX="22.0" layoutY="93.0" prefHeight="27.0" prefWidth="124.0" text="Name:" />
  <Label layoutX="22.0" layoutY="136.0" prefHeight="27.0" prefWidth="124.0" text="Address:" />
  <Label layoutX="22.0" layoutY="181.0" prefHeight="27.0" prefWidth="124.0" text="Age:" />
  <TextField fx:id="name" layoutX="106.0" layoutY="94.0" />
  <TextField fx:id="address" layoutX="106.0" layoutY="137.0" />
  <TextField fx:id="age" layoutX="106.0" layoutY="182.0" />
  <Button fx:id="add_btn" layoutX="127.0" layoutY="223.0" mnemonicParsing="false" onAction="#add_details" text="Add" />
  <TableView fx:id="table" layoutX="342.0" layoutY="51.0" prefHeight="260.0" prefWidth="305.0">
      <TableColumn fx:id="idcol" prefWidth="39.33331298828125" text="ID" />
      <TableColumn fx:id="namecol" prefWidth="111.3333740234375" text="Name" />
        <TableColumn fx:id="addresscol" prefWidth="110.00003051757812" text="Address" />
        <TableColumn fx:id="agecol" prefWidth="40.6666259765625" text="Age" />
  <Button fx:id="connect_db" layoutX="14.0" layoutY="310.0" mnemonicParsing="false" onAction="#connect" text="Connect DB" />
  <Button layoutX="200.0" layoutY="223.0" mnemonicParsing="false" onAction="#clear_data" text="Clear" />

I am not able to fetch the components form the .fxml file. this is my .fxml code

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