Why Flow cannot call ReactDOM.render with document.getElementById(...)

Aleksey L. got this first in the comments, I wanted to bring this info up to the answer level for easier visual scanning.

Flow is letting you know that the call document.getElementById("root"); can return null in which case the app would completely crash. So let's guard against that:

const root = document.getElementById('root')

if (root !== null) {
  ReactDOM.render(<App /> , root)
}

Granted, this can feel a little annoying given that in all likelihood you will be controlling the HTML you are rendering into.

While Cogell's answer is correct, I would argue to keep the code simpler and add an exception.

ReactDOM.render(
 <Provider store={createStoreWithMiddleware(reducers)}>
  <BrowserRouter>
    <ScrollToTop>
      <Switch>
        <Route exact path="/" component={Home}/>
        <Route path="/detail/:detailId" component={Detail}/>
        <Route path="/level-of-game" component={LevelOfGame}/>
        <Route path="*" component={NotFound} status={404}/>
      </Switch>
    </ScrollToTop>
  </BrowserRouter>
 </Provider>, // $FlowIgnore
 document.getElementById("root")
);

Notice the "$FlowIgnore" comment

and then in your .flowconfig file add this to the "options" field:

suppress_comment= \\(.\\|\n\\)*\\$FlowIgnore