A DESCRIPTION OF THE PROBLEM :
the document of the WebEngine return nulls after the PAGE_REPLACED event. Probably because the PAGE_REPLACED events set the boolean invalidate to false which causes the getDocument to always return null. 

REGRESSION : Last worked in version 8u162

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Load the following url in the WebEngine https://movies-finder.firebaseapp.com/ 
After clicking on one of the menu items ( which triggers the PAGE_REPLACED event), 
The document of the webEngine returns null


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The document of the webEngine is the same as Document e = (Document) engine.executeScript("document");
ACTUAL -
The document of the webEngine is null

---------- BEGIN SOURCE ----------
import javax.swing.JFrame;

import org.w3c.dom.Document;

import javafx.application.Platform;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.embed.swing.JFXPanel;
import javafx.scene.Scene;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebView;

public class TestCase_001 {
	public static void main(String[] args) {
		Platform.setImplicitExit(false);

		JFrame frame = new JFrame("JFXTest");
		frame.setSize(1400, 1000);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

		JFXPanel jfxPanel = new JFXPanel();

		Platform.runLater(() -> {
			WebView webView = new WebView();
			final WebEngine engine = webView.getEngine();
			engine.load("https://movies-finder.firebaseapp.com/");
			engine.documentProperty().addListener(new ChangeListener<Document>() {
				@Override
				public void changed(ObservableValue<? extends Document> prop, Document oldDoc, Document newDoc) {
					System.out.println("old document " + oldDoc);
					System.out.println("new document " + newDoc);

					Document e = (Document) engine.executeScript("document");
					Document e2 = engine.getDocument();
					System.out.println("document from script " + e);
					System.out.println("document from engine " + e2);
				}
			});
			Scene scene = new Scene(webView);
			jfxPanel.setScene(scene);
		});

		frame.add(jfxPanel);
		frame.setVisible(true);
	}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
 Object obj = engine.documentProperty();
            System.out.println(Arrays.toString(obj.getClass().getDeclaredMethods()));
            Method method = obj.getClass().getDeclaredMethod("invalidate",Boolean.TYPE);
            if (method!=null){
                method.setAccessible(true);
                method.invoke(obj, true);
            }

FREQUENCY : always