docs: public void addMainHandler(Object handler, String prefix) {
          if (handler == null) {
              throw new NullPointerException();
          }
          allHandlers.add(handler);
  
          addDeclaredMethods(handler, prefix);
          inputConverter.addDeclaredConverters(handler);
          outputConverter.addDeclaredConverters(handler);
  
          if (handler instanceof ShellDependent) {
              ((ShellDependent)handler).cliSetShell(this);
          }
      }