Problem with an example of code translation into ruby/js/kotlin/groovy

[Sammers21]

Take a look at the example of ruby/js/kotlin/groovy MQTT client usage code at documentation.

During the compile maven goal execution such error occurs when generating sources for ruby/js/kotlin/groovy:

Cannot generate VertxMqttClientExamples#example8 : It is not be possible to render an async result directly
Cannot generate VertxMqttClientExamples#example8 : It is not be possible to render an async result directly
Cannot generate VertxMqttClientExamples#example8 : It is not be possible to render an async result directly
Cannot generate VertxMqttClientExamples#example8 : It is not be possible to render an async result directly

How can it be fixed?