A DESCRIPTION OF THE PROBLEM :
When implementing Initializable in the JavaFX controller, the abstract method initialize should be @Override public void initialize(URL url, ResourceBundle rb){...} and not @Override public void initialize(URL url, Resource r){...}.

The full incorrect code in docs
package com.foo;

public class MyController implements Initializable {
    public Button button;

    @Override
    public void initialize(URL location, Resources resources)
        button.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                System.out.println("You clicked me!");
            }
        });
    }
}

What it should be.
package com.foo;

public class MyController implements Initializable {
    public Button button;

    @Override
    public void initialize(URL location, ResourceBundle resources)
        button.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                System.out.println("You clicked me!");
            }
        });
    }
}

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
View the docs at https://openjfx.io/javadoc/22/javafx.fxml/javafx/fxml/doc-files/introduction_to_fxml.html.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expected to see the following.

package com.foo;

public class MyController implements Initializable {
    public Button button;

    @Override
    public void initialize(URL location, ResourceBundle resources)
        button.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                System.out.println("You clicked me!");
            }
        });
    }
}
ACTUAL -
I currently see the following.

package com.foo;

public class MyController implements Initializable {
    public Button button;

    @Override
    public void initialize(URL location, Resources resources)
        button.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                System.out.println("You clicked me!");
            }
        });
    }
}