As indicated here, JavaFX is no longer included in openjdk.
So check, if you have <Java SDK root>/jre/lib/ext/jfxrt.jar on your classpath under Project Structure -> SDKs -> 1.x -> Classpath? If not, that could be why. Try adding it and see if that fixes your issue, e.g. on Ubuntu, install then openjfx package with sudo apt-get install openjfx.