reactjs - useReducer: userReducer を使用して初期状態を取得できません

Question

アクションタイプに基づいてアクションをディスパッチするためにuseReducerを実装しました。アクションタイプに基づいて状態変数を更新するたびに、初期状態を読み取ろうとすると常にエラーが発生します。おそらく私のコードはもっと説明できるでしょう。

initialLoginState.isLoading を読み取ろうとすると、エラーが発生します。

App.js コード:

import * as React from 'react';
import { NavigationContainer } from '@react-navigation/native';
import BottomNavigation from './src/Navigations/BottomNavigation';
import AuthStackNavigation from './src/Navigations/AuthStackNavigation'
import { useState, useEffect, useMemo, useReducer } from 'react';
import { View } from 'react-native-animatable';
import { ActivityIndicator } from 'react-native';
import { AuthenticationContext } from './src/context/AuthenticationContext'

export default function App() {

  //Initial state values
  const initialLoginState = {
    isLoading: false,
    userName: null,
    userToken: null
  }

//Reducer function
  const loginReducer = (action, prevState) => {
    switch (action.type) {
      case 'LOGIN':
        return {
          ...prevState,
          userToken: action.token,
          userName: action.id,
          isLoading: false
        };
      case 'LOGOUT':
        return {
          ...prevState,
          userName: null,
          userToken: null,
          isLoading: false,
        };
      case 'REGISTER':
        return {
          ...prevState,
          userToken: action.token,
          userName: action.id,
          isLoading: false,
        };
      case 'RETRIEVE_TOKEN ':
        return {
          ...prevState,
          userToken: action.token,
          isLoading: false
        };
    }
  }

  //Defining useReducer
  const [newLoginState, dispatch] = useReducer(loginReducer, initialLoginState);
  const authContext = useMemo(() => ({
    signIn: (email, password) => {
      // setUserToken('abc');
      // setIsLoading(false);

      console.log("called")
      let userToken
      userToken = null;
      if (email == 'user' && password == 'pass') {
        userToken = 'abc';
      }
      dispatch({ type: 'RETRIEVE_TOKEN', id: email, token: userToken })
    },
    signOut: () => {
      dispatch({ type: 'LOGOUT', })
    },
    signUp: () => {
      setUserToken(null);
      setIsLoading(false);
    }

  }), []);

  useEffect(() => {
    let userToken;
    userToken = "dfsdfsdf"
    dispatch({ type: 'REGISTER', token: userToken })
  }, [])

  if (initialLoginState.isLoading) {
    return (
      <View style={{ flex: 1, justifyContent: 'center', alignContent: 'center' }}>
        <ActivityIndicator size='large'></ActivityIndicator>
      </View>
    )
  }

  return (
    <AuthenticationContext.Provider value={authContext}>
      <NavigationContainer>
        {initialLoginState.userToken !== null ?
          <BottomNavigation />
          :
          <AuthStackNavigation />
        }
      </NavigationContainer>
    </AuthenticationContext.Provider>

  );
}

エラー：

TypeError: undefined is not an object (evaluating 'newLoginState.isLoading')
- node_modules/react-native/Libraries/LogBox/LogBox.js:148:8 in registerError
- node_modules/react-native/Libraries/LogBox/LogBox.js:59:8 in errorImpl
- node_modules/react-native/Libraries/LogBox/LogBox.js:33:4 in console.error