我正在构建一个flutter应用程序，我试图将一个图像上传到firebase数据库，但得到一个错误LateInitializationError: Field '_pickedImage' has not been initialized. Error: Unexpected null value.。我也使用?操作符检查了空值，但我不知道我错过了什么。

import 'dart:io';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_storage/firebase_storage.dart';
import 'package:get/get.dart';
import 'package:image_picker/image_picker.dart';

class AuthController extends GetxController {
  static AuthController instance = Get.find();
  late Rx<User?> _user;
  late Rx<File?> _pickedImage;

  File? get profilePhoto => _pickedImage.value;
  User get user => _user.value!;

  @override
  void onReady() {
    super.onReady();
    _user = Rx<User?>(firebaseAuth.currentUser);
    _user.bindStream(firebaseAuth.authStateChanges());
    ever(_user, _setInitialScreen);
  }

  _setInitialScreen(User? user) {
    if (user == null) {
      Get.offAll(() => LoginScreen());
    } else {
      Get.offAll(() => const HomeScreen());
    }
  }

  void pickImage() async {
    final pickedImage =
        await ImagePicker().pickImage(source: ImageSource.gallery);
    if (pickedImage != null) {
      Get.snackbar('Profile Picture',
          'You have successfully selected your profile picture!');
         }
    _pickedImage = Rx<File?>(File(pickedImage!.path));
  }

  // upload to firebase storage
  Future<String> _uploadToStorage(File image) async {
    Reference ref = firebaseStorage
        .ref()
        .child('profilePics')
        .child(firebaseAuth.currentUser!.uid);

    UploadTask uploadTask = ref.putFile(image);
    TaskSnapshot snap = await uploadTask;
    String downloadUrl = await snap.ref.getDownloadURL();
    return downloadUrl;
  }
 }