Javascript 任务管理器在后台的某个位置出错

我是react native的新手，我尝试在后台使用location，因此当用户进入他的家时，代码将执行一些操作。但我面临的错误是

    TaskManager: Task "locationTask" has been executed but looks like it is not defined. Please make sure that "TaskManager.defineTask" is called during initialization phase.

这是我的密码

export default function App(props) {
 var lat=0;
  var lng=0;
  users=firebase.auth().currentUser;
 if(users !=null){
 firebase.database().ref('mgnUsers/'+firebase.auth().currentUser.uid).once('value',(snap)=>{ 
 lat= snap.val().latitude;
  lng= snap.val().longitude;
   })
    try{
    if(!(lat==='0'&& lng==='0')){
Location.startGeofencingAsync('locationTask',[
    {
        "identifier": "A",
        "latitude": lat,//await AsyncStorage.getItem('latPoint'),
        "longitude": lng, //await AsyncStorage.getItem('lngPoint'),
        "radius":100,
        "notifyOnEnter": true,
        "notifyOnExit": true,

    }
   ])
   }
 }
//  }//end try
catch(error){
}//end catch
   }
   }

 TaskManager.defineTask('locationTask', async ({ data, error }) => {




 });