When using WebView, when running with JDK12 + OpenJFX12 Since ClassNotFoundException was issued, it is a memo of the countermeasure.
I added the following to [Run Configurations ...]-[Arguments]-[VM arguments] and the error disappeared.
--module-path "C:\Program Files\Java\javafx-sdk-12\lib"
--add-modules javafx.controls,javafx.web
--add-exports javafx.base/com.sun.javafx.logging=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.glass.ui=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.glass.utils=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.application=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.font=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.geom=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.geom.transform=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.scene=ALL-UNNAMED
--add-exports javafx.controls/com.sun.javafx.scene.control=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.scene.input=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.sg.prism=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.text=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.tk=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.javafx.util=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.prism=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.prism.paint=ALL-UNNAMED
--add-exports javafx.graphics/com.sun.scenario.effect=ALL-UNNAMED
https://www.reddit.com/r/JavaFX/comments/a3xjxy/how_to_run_controlsfx_java_11_and_javafx_11/ https://stackoverflow.com/questions/53215814/javafx11-java-lang-classnotfoundexception-com-sun-media-jfxmedia-events-players/53275389?noredirect=1#comment93434728_53275389