If you are using ControlsFX 11, add the following VM option to your runtime command line:
--add-exports javafx.base/com.sun.javafx.event=org.controlsfx.controls
Note: Previous answers (on other sites and also here on SO) have suggested the following:
--add-exports javafx.base/com.sun.javafx.event=ALL-UNNAMED
However, recent versions of ControlFX appear to be modularized and the modules are no longer unnamed. If you are using a previous version (say, version 9), use the ALL-UNNAMED variant of the command line option.