MyClass cannot access class com.sun.javafx.tk.Toolkit because module javafx.graphics does not export com.sun.javafx.tk to unnamed module

Question

I run my JavaFX app with VM options:

--module-path "C:\Software\javafx-sdk-18.0.1\lib" --add-modules javafx.base,javafx.controls,javafx.fxml,javafx.graphics

yet when I try to run the app, I get:

Caused by: java.lang.IllegalAccessError: class MyWindow (in unnamed module @0xb083ee4) cannot access class com.sun.javafx.tk.Toolkit (in module javafx.graphics) because module javafx.graphics does not export com.sun.javafx.tk to unnamed module @0xb083ee4

(As a small note, I rely on FontMetrics, if this makes difference.)

I am wondering what's going on and how to fix it?