How do I get Java FX running with OpenJDK 8 on Ubuntu 18.04.2 LTS?

The hint from

https://github.com/JabRef/help.jabref.org/issues/204

helped. The commands:

apt purge openjfx
apt install openjfx=8u161-b12-1ubuntu2 libopenjfx-jni=8u161-b12-1ubuntu2 libopenjfx-java=8u161-b12-1ubuntu2
apt-mark hold openjfx libopenjfx-jni libopenjfx-java

fixed the problem. Many thanks to https://github.com/Druidefix for this. (If you'd like to answer this yourself I'll happily delete my own answer ...)

What I did was build it from source. The instructions here on the OpenJDK wiki worked for me. Once I built it, I had to put the jars in the right place, like this other stackoverflow post talked about.

cd build/sdk/
sudo cp -R rt/lib /usr/lib/jvm/java-8-openjdk-amd64/jre/lib/
sudo cp ./lib/ant-javafx.jar /usr/lib/jvm/java-8-openjdk-amd64/lib/
sudo cp ./lib/javafx-mx.jar /usr/lib/jvm/java-8-openjdk-amd64/lib/

After that I was able to compile my project!