When an onSubscribe hook is installed and returns a null consumer, the current error report says:

ObjectHelper.requireNonNull(observer, "Plugin returned null Observer");

At least an "a" is missing but perhaps it would be better to word it more detailed:

"The RxJavaPlugins.onSubscribe hook returned a null Observer. Please check the handler provided to RxJavaPlugins.setOnObservableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins"

Of course, all reactive types should be updated and adapted to the appropriate types and method names.