How can I enable a Dialog's button from an event handler method?

[Jj Roberts]

I'm trying to learn a sprinkling of JavaFx to test the backend code of a personal project I am working on. At the moment I am trying to create a simple login dialog. I want to disable the 'ok' button until all the input fields contain valid data. I have knocked together a little example to demonstrate what I want.

For context, this is how I create the dialog:

@FXML
public void buttonClicked() throws Exception {
    URL url = getClass().getClassLoader().getResource(DIALOG);
    FXMLLoader loader = new FXMLLoader(url);
    if (url == null) {
        throw new RuntimeException("Can't load fxml");
    }
    Parent root = loader.load();

    Dialog<ButtonType> dialog = new Dialog<>();
    dialog.getDialogPane().setContent(root);
    dialog.getDialogPane().getButtonTypes().addAll(ButtonType.CANCEL, ButtonType.OK);
    dialog.getDialogPane().lookupButton(ButtonType.OK).setDisable(true);
    dialog.showAndWait();
}

and this is the fxml:

<DialogPane fx:controller="sample.Controller" xmlns="http://javafx.com/javafx/10.0.2-internal" xmlns:fx="http://javafx.com/fxml/1">
   <content>
      <TextField fx:id="textField" onKeyReleased="#validateInput"/>
   </content>
</DialogPane>

My issue is enabling the button once the input it valid. I have another method 'validateInput()' to check if the TextField contains valid data.

@FXML
public void validateInput() {
    String text;
    if (textField != null) {
        text = textField.getText();
        if (!text.isBlank()) {
            //re-enable the ok button here
        }
    }
}

I tried creating a field for the DialogPane, but calling lookUpButton(ButtonType.OK) always returns null. I also tried using a Dialog<ButtonType> field and then setting it to the Dialog object I create in buttonClicked(), instead of a local variable:

myDialogField = new Dialog<>();
dialog.getDialogPane().setContent(root);
dialog.getDialogPane().getButtonTypes().addAll(ButtonType.CANCEL, ButtonType.OK);
dialog.getDialogPane().lookupButton(ButtonType.OK).setDisable(true);
dialog.showAndWait();

The field is always null from the validateInput() method, however.

I'm more or less shooting in the dark here. There is a lot to JavaFx which I will have to get to grips with
How can I enable the button from the validateInput() method?