I can't get the text from an Dialog Alert from the Test Class

Good morning, i am doing a test class for one javafx class, im trying to control errors right now. When an error ocurred shows me a Dialog Alert with a message who explain the error. I want to compare this message with other in my test class.
Thats last thing that i try.

LOGIN CLASS

catch (ServerConnectionErrorException ex) {
            LOGGER.warning("LoginWindowController: Server connection error");
            Alert alert = new Alert(Alert.AlertType.ERROR);
            alert.setTitle("Server Error");
            alert.setContentText("Unable to connect with server");
            Label serverErrorContext = new Label();
            serverErrorContext.setText(alert.getContentText());
            serverErrorContext.setId("serverErrorContext");
            alert.showAndWait();
        }

TEST CLASS

@Test
    public void test4_ConnectionError(){
        clickOn("#txtLogin");
        write("user");
        clickOn("#txtPass");
        write("BBccd1234");
        clickOn("#btLogin");
        FxAssert.verifyThat("#serverErrorContext",LabeledMatchers.hasText("Unable to connect with server"));
        clickOn(".button");

    }