TestFX cannot localize node during Travis CI build after tested application was opened another window

Question

I am working on simple java standalone application with GUI created with JavaFX 8.

I wrote some tests of GUI with TestFX framework (version 3.1.0). On my local machine tests works fine, but they are failed on Travis CI environment. Please take a look, some fragments of code:

import static org.assertj.core.api.Assertions.assertThat;
import static org.loadui.testfx.GuiTest.find;

public class JavafxApplicationIT {
    private static GuiTest controller;

    @BeforeClass
    public static void setAfter() throws Exception {
        FXTestUtils.launchApp(JavafxApplication.class);
        Thread.sleep(5000);
        controller = new GuiTest() {
            @Override
            protected Parent getRootNode() {
                return JavafxApplication.getScene().getRoot();
            }
        };
    }

    @AfterClass
    public static void shutdownAll() throws InterruptedException {
        Platform.runLater(() -> JavafxApplication.getStage().close());
    }

    @Test
    public void shouldFillForm_saveLicence_thenOpen_andVerifySignature() throws Throwable {
        controller.click("#newLicenceButton");

        ((TextField) find("#licenceUidField")).setText(licenceUID);
        ((TextField) find("#customerIdField")).setText(customerId);
        ....
        controller.click("#mainActionButton");
        Thread.sleep(3000);
        ((TextField) find("#licFileField")).setText(licenceFile.getPath()); // <--- FAILED

    }
}

I've got No nodes matched '#licFileField' error. I notice that in every failing cases problem with matching is always after opening some new window in application.

In above case, clicking on mainActionButton invokes new window, and licFileField is a name of element from new (active) window.

Maybe it is connected with configuration of xvfb on Travis CI machine? In .travis.yml I've got:

before_install:
  - "export DISPLAY=:99.0"
  - "sh -e /etc/init.d/xvfb start"
  - "/sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac -screen 0 1280x1024x16"