ADDITIONAL SYSTEM INFORMATION :
OS: Ubuntu 14.04 / 18.04 / Windows 10 Pro 1803
Java: Java 8 Update 172/181

A DESCRIPTION OF THE PROBLEM :
When I print WebView by PrinterJob.printPage(Node), IllegalStateException is thrown. the page is printed omitting checkboxes. Printing by WebEngine.print(PrinterJob) results in the same.
A similar problem is reported in
https://stackoverflow.com/questions/50476171/illegalstateexeption-when-using-javafx-webengine-print-function

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Load a HTML page by WebView in JavaFX. The HTML contains checkboxes.
Print the WebView by PrinterJob.printPage(Node).

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The WebView is printed as shown on the screen.
ACTUAL -
The WebView is printed, but checkboxes is not printed. And IllegalStateException is thrown as follows:

Exception in thread "JavaFX Application Thread" java.lang.IllegalStateException: the method should not be called in this context
at com.sun.javafx.webkit.theme.RenderThemeImpl.ensureNotDefault(RenderThemeImpl.java:250)
at com.sun.javafx.webkit.theme.RenderThemeImpl.createWidget(RenderThemeImpl.java:263)
at com.sun.webkit.WebPage.twkUpdateContent(Native Method)
at com.sun.webkit.WebPage.lambda$print$40(WebPage.java:666)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.gtk.GtkApplication.enterNestedEventLoopImpl(Native Method)
at com.sun.glass.ui.gtk.GtkApplication._enterNestedEventLoop(GtkApplication.java:211)
at com.sun.glass.ui.Application.enterNestedEventLoop(Application.java:511)
at com.sun.glass.ui.EventLoop.enter(EventLoop.java:107)
at com.sun.javafx.tk.quantum.QuantumToolkit.enterNestedEventLoop(QuantumToolkit.java:590)
at com.sun.prism.j2d.print.J2DPrinterJob$J2DPageable.implPrintPage(J2DPrinterJob.java:1146)
at com.sun.prism.j2d.print.J2DPrinterJob$J2DPageable.access$500(J2DPrinterJob.java:972)
at com.sun.prism.j2d.print.J2DPrinterJob.print(J2DPrinterJob.java:797)
at javafx.print.PrinterJob.printPage(PrinterJob.java:398)
at javafx.print.PrinterJob.printPage(PrinterJob.java:414)
at bugreport.Main.print(Main.java:67)
at bugreport.Main.lambda$2(Main.java:45)
at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at com.sun.glass.ui.gtk.GtkApplication.lambda$null$48(GtkApplication.java:139)
at java.lang.Thread.run(Thread.java:748)
Exception in thread "JavaFX Application Thread" java.lang.IllegalStateException: the method should not be called in this context
at com.sun.javafx.webkit.theme.RenderThemeImpl.ensureNotDefault(RenderThemeImpl.java:250)
at com.sun.javafx.webkit.theme.RenderThemeImpl.createWidget(RenderThemeImpl.java:263)
at com.sun.webkit.WebPage.twkUpdateContent(Native Method)
at com.sun.webkit.WebPage.lambda$print$40(WebPage.java:666)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.gtk.GtkApplication.enterNestedEventLoopImpl(Native Method)
at com.sun.glass.ui.gtk.GtkApplication._enterNestedEventLoop(GtkApplication.java:211)
at com.sun.glass.ui.Application.enterNestedEventLoop(Application.java:511)
at com.sun.glass.ui.EventLoop.enter(EventLoop.java:107)
at com.sun.javafx.tk.quantum.QuantumToolkit.enterNestedEventLoop(QuantumToolkit.java:590)
at com.sun.prism.j2d.print.J2DPrinterJob$J2DPageable.implPrintPage(J2DPrinterJob.java:1146)
at com.sun.prism.j2d.print.J2DPrinterJob$J2DPageable.access$500(J2DPrinterJob.java:972)
at com.sun.prism.j2d.print.J2DPrinterJob.print(J2DPrinterJob.java:797)
at javafx.print.PrinterJob.printPage(PrinterJob.java:398)
at javafx.print.PrinterJob.printPage(PrinterJob.java:414)
at bugreport.Main.print(Main.java:67)
at bugreport.Main.lambda$2(Main.java:45)
at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at com.sun.glass.ui.gtk.GtkApplication.lambda$null$48(GtkApplication.java:139)
at java.lang.Thread.run(Thread.java:748)


---------- BEGIN SOURCE ----------
------Main.java----------------
package bugreport;

import java.util.concurrent.CountDownLatch;

import javafx.application.Application;
import javafx.application.Platform;
import javafx.beans.value.ChangeListener;
import javafx.concurrent.Worker.State;
import javafx.print.Printer;
import javafx.print.PrinterJob;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebView;


public class Main extends Application {
@Override
public void start(Stage primaryStage) {
try {
WebView root = new WebView();
WebEngine engine = root.getEngine();
engine.load(getClass().getResource("BugReport.html").toString());

CountDownLatch latch = new CountDownLatch(1);
ChangeListener<State> loadFinish = (observable, oldValue, newValue) -> {
if (newValue == State.SUCCEEDED) {
latch.countDown();
}
};
engine.getLoadWorker().stateProperty().addListener(loadFinish);

Scene scene = new Scene(root,400,400);
primaryStage.setScene(scene);
primaryStage.show();

new Thread(() -> {
// wait the html loaded.
try {
latch.await();
} catch (InterruptedException e) {
}
Platform.runLater(() -> {
print(root);
/*
PrinterJob job = PrinterJob.createPrinterJob(Printer.getDefaultPrinter());
engine.print(job);
job.endJob();
*/
});
}).start();

} catch(Exception e) {
e.printStackTrace();
}
}

public static void main(String[] args) {
launch(args);
}

private static void print(WebView node) {
Printer printer = Printer.getDefaultPrinter();
if (printer != null) {
PrinterJob job = PrinterJob.createPrinterJob(printer);
try {
job.printPage(node);
} finally {
job.endJob();
}
}
}
}
--------End of Main.java-----------------------------------------------------------
-----------BugReport.html----------------------------------------
<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
    <title>BugReport</title>
  </head>
  <body><input name="bugreport" type="checkbox">
    <p>This sentence is printed.<br>
    </p>
  </body>
</html>
---------End of BugReport.html--------------------
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
In my case, no exception is thrown when checkboxes are removed from the HTML.
But the checkboxes are not printed, of course.

FREQUENCY : always