javascript - React TypeError：调度不是一个函数

Question

请帮助我，我的代码完全搞砸了，我收到了这个错误，我无法解决它。

我知道这个问题有几个答案，但它们并没有解决我的问题。我是新手，无法找到正确的代码。

    19 | useEffect(() => {
    20 |   const user = 
    JSON.parse(localStorage.getItem("user"));
    21 |   if (user) {
  > 22 |     dispatch({type:"USER",payload:user})
 ^  23 |   } else {
    24 |     history.push("/signin");
    25 |   }

  App.js 

import React, { useEffect, createContext, useReducer, useContext } from "react";
import Navbar from "./components/Navbar";
import "./App.css";
import { BrowserRouter, Route, Switch, useHistory } from "react-router-dom";
import Home from "./components/screens/Home";
import Signin from "./components/screens/Signin";
import Signup from "./components/screens/Signup";
import Profile from "./components/screens/Profile";
import CreatePost from "./components/screens/CreatePost";
import UserProfile from "./components/screens/UserProfile";
// import Hero from "./components/screens/Hero";
import { reducer, initialState } from "./reducers/userReducer";

export const UserContext = createContext();

const Routing = () => {
  const history = useHistory();
  const { state, dispatch } = useContext(UserContext);
  useEffect(() => {
    const user = JSON.parse(localStorage.getItem("user"));
    if (user) {
      dispatch({type:"USER",payload:user})
    } else {
      history.push("/signin");
    }
  }, []);
  return (
    <Switch>
      <Route exact path="/">
        <Home />
      </Route>
      {/* <Route exact path="/post">
        <Hero />
      </Route> */}
      <Route path="/signin">
        <Signin />
      </Route>
      <Route path="/signup">
        <Signup />
      </Route>
      <Route exact path="/profile">
        <Profile />
      </Route>
      <Route path="/create">
        <CreatePost />
      </Route>
      <Route path="/profile/:userid">
        <UserProfile />
      </Route>
    </Switch>
  );
};

function App() {
  const [state, dispatch] = useReducer(reducer, initialState);
  return (
    <UserContext.Provider value={(state, dispatch)}>
      <BrowserRouter>
        <Navbar />
        <Routing />
      </BrowserRouter>
    </UserContext.Provider>
  );
}

export default App;

---

userReducer.js

export const initialState = null;

export const reducer = (state, action) => {
  if (action.type === "USER") {
    return action.payload;
  }
  if (action.type === "CLEAR") {
    return null;
  }
  if (action.type === "UPDATE") {
    return {
      ...state,
      followers: action.payload.followers,
      following: action.payload.following,
    };
  }
  return state;
};