NullPointerException when Setting MFXTextField's Text to Null

[gilad4]

Issue Description

There is a NullPointerException (NPE) thrown when changing the text of an MFXTextField to null. This issue occurs due to missing null checks in the listeners within the MFXTextFieldSkin class.

Steps to Reproduce

1. Create a JavaFX application with an MFXTextField instance
2. Initialize the MFXTextField's text with a non-null value.
3. Add the MFXTextField to a scene and show the scene on a stage.
4. Set the text of the MFXTextField to any non-null value (e.g., textField.setText("Initial Value")).
5. Then, set the text of this MFXTextField to null (e.g., textField.setText(null)).
6. Observe the NullPointerException. (On console or with Thread.currentThread().setUncaughtExceptionHandler(...) )

MRE

import io.github.palexdev.materialfx.controls.MFXTextField;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.scene.Scene;
import javafx.scene.control.Alert;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;

public class MFXTextFieldNPEExample extends Application {

    @Override
    public void start(Stage primaryStage) {
        // Set an uncaught exception handler to show a popup
        Thread.currentThread().setUncaughtExceptionHandler((thread, throwable) -> {
            Platform.runLater(() -> {
                Alert alert = new Alert(Alert.AlertType.ERROR);
                alert.setContentText(throwable.toString());
                alert.showAndWait();
            });
        });

        // Create the MFXTextField with an initial non-null text value
        MFXTextField textField = new MFXTextField("Initial Value");

        // Create the scene and add the textField
        Scene scene = new Scene(new Pane(textField), 300, 200);

        // Set the scene and show the stage
        primaryStage.setScene(scene);
        primaryStage.setTitle("MFXTextField NPE Example");
        primaryStage.show();

        // Set the text of the MFXTextField to null to reproduce the issue
        textField.setText(null);
    }

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

Expected Behavior

The MFXTextField should handle null values for text without throwing an exception, perhaps treating them as empty strings.

Actual Behavior

Setting the text to null causes a NullPointerException in the addListeners method of the MFXTextFieldSkin class.

Proposed Solution

A simple null check can be added to the listeners in the addListeners method. Here is the problematic code snippet:

textField.promptTextProperty().addListener((observable, oldValue, newValue) -> {
    if (!newValue.isEmpty() && !isFloating()) textField.requestLayout();
});
textField.textProperty().addListener((observable, oldValue, newValue) -> {
    if (!newValue.isEmpty() && !isFloating()) textField.requestLayout();
});