As of writing, VisualVM 1.4.2 is the newest version of VVM available. This doesn’t currently run against Java 12, and will bomb with an error dialogue if you try.
If you’ve installed Java 12 on Mac, it will be the ‘default version’ and VVM won’t start.
It can be started against a specified JDK by adding it to the command line:
/Applications/VisualVM.app/Contents/MacOS/visualvm \ --jdkhome /Library/Java/JavaVirtualMachines/jdk-11.jdk/Contents/Home
I haven’t tried connecting it to JDK12 apps yet, so YMMV.