#vulnerable code 
                              @Override
  public void handle(final Message<JsonObject> message) {
    String action = getMandatoryString("action", message);

    if (action == null) {
      sendError(message, "An action must be specified.");
    }

    switch (action) {
      case "register":
        doRegister(message);
        break;
      case "assign":
        doAssign(message);
        break;
      case "release":
        doRelease(message);
        break;
      default:
        sendError(message, String.format("Invalid action %s.", action));
    }
  } 
                            #location 9 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Override
  public void handle(final Message<JsonObject> message) {
    String action = getMandatoryString("action", message);

    if (action == null) {
      sendError(message, "An action must be specified.");
      return;
    }

    switch (action) {
      case "register":
        doRegister(message);
        break;
      case "assign":
        doAssign(message);
        break;
      case "release":
        doRelease(message);
        break;
      default:
        sendError(message, String.format("Invalid action %s.", action));
    }
  }