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One way to make a TextButton inactive when the text field has no text is to use a listener on the text field to update the TextButton's enabled property.

Here is an example:

TextField textField = new TextField();
TextButton button = new TextButton("Submit");

textField.addListener(new InputListener() {
    @Override
    public void keyTyped(InputEvent event, char character) {
        if (textField.getText().isEmpty()) {
            button.setDisabled(true);
        } else {
            button.setDisabled(false);
        }
    }
});

button.setDisabled(true);

In this example, we add a listener to the text field that checks whether the text field's text is empty. If it is, we disable the TextButton. If it is not, we enable the TextButton.

We also start by setting the TextButton's enabled property to false, since the text field starts out empty.