I had the same problem, in my case i resolved it by:
1) going to File-->Project Structure---->Global libraries 2) looking for jfxrt.jar included as default in the jdk1.8.0_241\lib (after installing it) 3)click on + on top left to add new global library and i specified the path of my jdk1.8.0_241 Ex :(C:\Program Files\Java\jdk1.8.0_241).
I hope this will help you