index.js를 다음과 같이 수정해 보세요.

src/index.js

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import * as serviceWorker from './serviceWorker';
import { BrowserRouter } from 'react-router-dom';
import { Provider } from 'react-redux';
import { createStore, applyMiddleware } from 'redux';
import { composeWithDevTools } from 'redux-devtools-extension';
import createSagaMiddleware from 'redux-saga';
import rootReducer, { rootSaga } from './modules';
import { tempSetUser, check } from './modules/user';

const sagaMiddleware = createSagaMiddleware();
const store = createStore(
  rootReducer,
  composeWithDevTools(applyMiddleware(sagaMiddleware)),
);

function loadUser() {
  try {
    const user = localStorage.getItem('user');
    if (!user) return; // 로그인 상태가 아니라면 아무것도 안 함

    store.dispatch(tempSetUser(user));
    store.dispatch(check());
  } catch (e) {
    console.log('localStorage is not working');
  }
}

sagaMiddleware.run(rootSaga);
loadUser();

ReactDOM.render(
  <Provider store={store}>
    <BrowserRouter>
      <App />
    </BrowserRouter>
  </Provider>,
  document.getElementById('root'),
);

serviceWorker.unregister();

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