Going through the tutorial to deploy a REST API extension:
https://documentation.bonitasoft.com/bonita/2021.1/rest-api-extensions
On completion, calling the service had no data in the response. Looking to the logs, found the following:
INFO (http-nio-8080-exec-1) org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/bonita] RestletServlet: [Restlet] ServerServlet: component class is null
Found an already existing question: