Looks like this is real bug! Executor should be registered in BrwsrCtx, but it is not: you can workaround by building new context yourself and putting all the technologies there + the Executor.

http://hg.netbeans.org/html4j/rev/e390a06dbfac

The fix will be part of 1.3 version.

Here is fix of your code that works with current version 1.2.3:

--- a/src/test/java/com/dukescript/test/selenium/webdriver/ExternalWebView.java
+++ b/src/test/java/com/dukescript/test/selenium/webdriver/ExternalWebView.java
@@ -22,7 +22,6 @@
  * <http://www.gnu.org/licenses/gpl-3.0.html>.
  * #L%
  */
-import java.util.List;
 import java.util.concurrent.CountDownLatch;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -48,12 +47,17 @@
     @BeforeClass
     public static void test() throws InterruptedException, Exception {
         final CountDownLatch countDownLatch = new CountDownLatch(1);
-
+        final WebView[] arr = { null };
         final Runnable done = new Runnable() {
             @Override
             public void run() {
                 testModel = new TestModel("Hello", "World");
                 testModel.applyBindings();
+                try {
+                    driver = new WebDriverFX(arr[0]);
+                } catch (Exception ex) {
+                    Logger.getLogger(ExternalWebView.class.getName()).log(Level.SEVERE, null, ex);
+                }
                 countDownLatch.countDown();
             }
         };
@@ -63,11 +67,11 @@
                 try {
                     Stage stage = new Stage();
                     final WebView webView = new WebView();
+                    arr[0] = webView;
                     Scene scene = new Scene(webView);
                     stage.setScene(scene);
                     stage.show();
                     FXBrowsers.load(webView, ExternalWebView.class.getResource("testWithModel.html"), done);
-                    driver = new WebDriverFX(webView);
                 } catch (Exception ex) {
                     Logger.getLogger(ExternalWebView.class.getName()).log(Level.SEVERE, null, ex);
                 }