Was not able to find the cause of the problem -- all of the DiscoveryProvider subclasses check for null before calling this listener method. I added a warn log message with a stack trace in the event ...
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o. java.lang.NullPointerException at java.base/jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance ...