If you want to use JavaFX with OpenJDK 8 on Ubuntu you have to install an older version of OpenJFX and its dependencies.

sudo apt install openjdk-8-jdk openjfx=8u161-b12-1ubuntu2 libopenjfx-java=8u161-b12-1ubuntu2 libopenjfx-jni=8u161-b12-1ubuntu2

Hold the package, so they wont be upgraded on system updates.

sudo apt-mark hold openjfx libopenjfx-java libopenjfx-jni

Tested on Ubuntu 18.04.2