Cannot read property 'getState' of undefined error

You need to either pass a store to the Provider as Mark suggests, or if you have simplified your example to the point of not needing it, then remove the Provider element entirely so you are just rendering the App element. The current stack trace shows that the error is in Provider.

You also need to add:

export default App;

to the bottom of App.js.

You need to pass your store as props to the wrapping Provider

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