在playframework2.0中使用akka promise/future的例外情况

我使用PlayFramework2.0作为一个简单的应用程序，该应用程序从MySQL数据库返回json字符串形式的员工列表。我的控制器如下

public class Application extends Controller {

 public static Result getEmployees() {
        Logger.info("enter getting employee information");
        Promise <List<Employee>> employeeList = Akka.future(new     Callable<List<Employee>>(){
            public List<Employee> call() {
                return Employee.getAll();

            }
        });

        return async(
                  employeeList.map(
                  new Function<List<Employee>,Result>() {
                    @Override
                    public Result apply(List<Employee> employeeList)
                             {
                        // TODO Auto-generated method stub
                      try { 
                        if (employeeList!=null) {
                            return ok(Json.toJson(employeeList));
                        } else {
                            return ok("");
                        }
                      } catch (Exception e) {
                          return ok("");
                      }
                    } 
                  }
                )
         );

  }
}

我得到以下错误

[error] play - Waiting for a promise, but got an error: null
java.lang.RuntimeException: null

我的Akka特定配置是

play {

    akka {
        event-handlers = ["akka.event.slf4j.Slf4jEventHandler"]
        loglevel = WARNING

        actor {

            deployment {

                /actions {
                    router = round-robin
                    nr-of-instances = 100
                }

                /promises {
                    router = round-robin
                    nr-of-instances = 100
                }

            }

            retrieveBodyParserTimeout = 4 second

            actions-dispatcher = {
                fork-join-executor {
                    parallelism-factor = 1
                    parallelism-max = 100
                }
            }

            promises-dispatcher = {
                fork-join-executor {
                    parallelism-factor = 1
                    parallelism-max = 100
                }
            }

            websockets-dispatcher = {
                fork-join-executor {
                    parallelism-factor = 1
                    parallelism-max = 100
                }
            }

            default-dispatcher = {
                fork-join-executor {
                    parallelism-factor = 1
                    parallelism-max = 100
                }
            }

        }

    }

}

[error] play - Waiting for a promise, but got an error: null
java.lang.RuntimeException: null

play {

    akka {
        event-handlers = ["akka.event.slf4j.Slf4jEventHandler"]
        loglevel = WARNING

        actor {

            deployment {

                /actions {
                    router = round-robin
                    nr-of-instances = 100
                }

                /promises {
                    router = round-robin
                    nr-of-instances = 100
                }

            }

            retrieveBodyParserTimeout = 4 second

            actions-dispatcher = {
                fork-join-executor {
                    parallelism-factor = 1
                    parallelism-max = 100
                }
            }

            promises-dispatcher = {
                fork-join-executor {
                    parallelism-factor = 1
                    parallelism-max = 100
                }
            }

            websockets-dispatcher = {
                fork-join-executor {
                    parallelism-factor = 1
                    parallelism-max = 100
                }
            }

            default-dispatcher = {
                fork-join-executor {
                    parallelism-factor = 1
                    parallelism-max = 100
                }
            }

        }

    }

}