Cannot read property 'user' of undefined, React Redux

I have been getting this error "Cannot read property 'user' of undefined" in a userSlice.js which by default is named as counterSlice.js in react-redux. I have tried exporting by changing names and function names too, and I guess I am exporting the right function.

any suggestions or fix that might get it running?

Here is my userSlice.js file,

import { createSlice } from '@reduxjs/toolkit';
export const userSlice = createSlice({
  name: 'user',
  initialState: {
    user: null,
  },
  reducers: {
    login: (state, action) => {
      state.user = action.payload;
    },
    logout: (state) => {
      state.user = null;
    },
  },
});

export const { login, logout} = userSlice.actions;
export const selectUser = state => state.user.user;
export default userSlice.reducer;

here is the store.js file,

import { configureStore } from '@reduxjs/toolkit';
import userReducer from '../features/userSlice';

export default configureStore({
  reducer: {
    counter: userReducer,
  },
});

and here is the App.js file where I am trying to make the login user state to be logged in,

import React, {useEffect} from 'react';
import './App.css';
import Homescreen from "./screens/Homescreen";
import LoginScreen from "./screens/LoginScreen";
import {auth} from "./firebase";
import {useDispatch, useSelector} from "react-redux";
import {login, logout, selectUser} from "./features/userSlice";
import {BrowserRouter as Router, Switch,Route} from 'react-router-dom';


function App() {
    const user = useSelector(selectUser);
    const dispatch = useDispatch();

    useEffect(() => {
        const unsubscribe = auth.onAuthStateChanged((userAuth) => {
            if(userAuth){
                dispatch(login({
                    uid: userAuth.uid,
                    email: userAuth.email,
                })
                );
            } else{
                dispatch(logout())
            }
        });

        return unsubscribe;
    }, []);

  return (
    <div className="App">
        <Router>
            {!user ? (
                <LoginScreen/>
            ):(
            <Switch>
                <Route exact path="/">
                    <Homescreen />
                </Route>
            </Switch>
            )}
        </Router>
    </div>
  );
}

export default App;