"Package javafx.application is declared in module javafx.fxml, which is not in the module graph"

So, looks like the javafx.application package is not in the Java 9 module path. You can add it to module path by adding --add-modules javafx.application in File | Settings | Build, Execution, Deployment | Compiler | Java Compiler | Javac Options | Additional command line parameters:

or add require directive in your java module.info configuration file. See also example here https://github.com/ozlerhakan/java9-module-examples/tree/master/javafx#javafx about

 (package javafx.application is declared in module javafx.graphics, but module com.example.javafx does not read it)

error.

If issue remains, provide a sample project.

 ThankYou, Andrey, solution of the issue was simpler than I thought.