When the statement below is called outside of the FX application thread:

        new HTMLEditor().getHtmlText();

the following exception is raised:

SEVERE: Failed to load skin 'com.sun.javafx.scene.web.skin.HTMLEditorSkin' for control HTMLEditor@2e641d07
java.lang.IllegalStateException: Not on FX application thread; currentThread = LibraryFolderWatcher(/Users/elp/Library/Application Support/Scene Builder/Library)
at com.sun.javafx.tk.Toolkit.checkFxUserThread(Toolkit.java:204)
at com.sun.javafx.tk.quantum.QuantumToolkit.checkFxUserThread(QuantumToolkit.java:421)
at javafx.scene.web.WebEngine.checkThread(WebEngine.java:1182)
at javafx.scene.web.WebEngine.<init>(WebEngine.java:822)
at javafx.scene.web.WebEngine.<init>(WebEngine.java:811)
at javafx.scene.web.WebView.<init>(WebView.java:271)
at com.sun.javafx.scene.web.skin.HTMLEditorSkin.<init>(HTMLEditorSkin.java:271)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
flect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at javafx.scene.control.Control.loadSkinClass(Control.java:722)
at javafx.scene.control.Control$4.invalidated(Control.java:659)
at javafx.beans.property.StringPropertyBase.markInvalid(StringPropertyBase.java:109)
at javafx.beans.property.StringPropertyBase.set(StringPropertyBase.java:144)
at javafx.css.StyleableStringProperty.set(StyleableStringProperty.java:83)
at javafx.scene.control.Control$4.set(Control.java:651)
at javafx.css.StyleableStringProperty.applyStyle(StyleableStringProperty.java:69)
at javafx.css.StyleableStringProperty.applyStyle(StyleableStringProperty.java:45)
at javafx.scene.web.HTMLEditor.<init>(HTMLEditor.java:54)
at custom.CustomPaneController.initialize(CustomPaneController.java:11)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2570)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2445)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2413)
at custom.CustomPane.load(CustomPane.java:19)
at custom.CustomPane.<init>(CustomPane.java:10)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at java.lang.Class.newInstance(Class.java:437)
at sun.reflect.misc.ReflectUtil.newInstance(ReflectUtil.java:51)
at javafx.fxml.FXMLLoader$InstanceDeclarationElement.constructValue(FXMLLoader.java:1005)
at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:742)
at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2711)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2531)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2429)
at com.oracle.javafx.scenebuilder.kit.library.util.JarExplorer.instantiateWithFXMLLoader(JarExplorer.java:105)
at com.oracle.javafx.scenebuilder.kit.library.util.JarExplorer.exploreEntry(JarExplorer.java:146)
at com.oracle.javafx.scenebuilder.kit.library.util.JarExplorer.explore(JarExplorer.java:65)
at com.oracle.javafx.scenebuilder.kit.library.user.LibraryFolderWatcher.exploreAndUpdateLibrary(LibraryFolderWatcher.java:299)
at com.oracle.javafx.scenebuilder.kit.library.user.LibraryFolderWatcher.runDiscovery(LibraryFolderWatcher.java:122)
at com.oracle.javafx.scenebuilder.kit.library.user.LibraryFolderWatcher.run(LibraryFolderWatcher.java:88)
at java.lang.Thread.run(Thread.java:745)