Open Terminal and execute this (without the quotes): "/usr/libexec/java_home -v 1.8"
You have to see the Java home path (something line this: /Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home)
Copy it and then execute this: /Applications/OracleDataModeler.app/Contents/Resources/datamodeler/datamodeler/bin/datamodeler
It will ask for Java home path, paste it and hit Enter. Data Modeler will start normally.
That's it. Next time you will be able to open Data Modeler as usual.