TypeError: this.input.focus is not a function

This is the simplest app that produces this error (generated using `create-react-app`):

```es6
import React, { Component } from 'react';
import {TextInput} from 'react-desktop/macOs'

class App extends Component {
focus() {
this.input.focus();
}

render() {
return (

When I click the button I get:

```
App.js:8 Uncaught TypeError: this.input.focus is not a function
at App.focus (App.js:8)
at onClick (App.js:15)
at HTMLUnknownElement.callCallback (react-dom.development.js:540)
at Object.invokeGuardedCallbackDev (react-dom.development.js:579)
at Object.invokeGuardedCallback (react-dom.development.js:436)
at Object.invokeGuardedCallbackAndCatchFirstError (react-dom.development.js:450)
at executeDispatch (react-dom.development.js:834)
at executeDispatchesInOrder (react-dom.development.js:856)
at executeDispatchesAndRelease (react-dom.development.js:954)
at executeDispatchesAndReleaseTopLevel (react-dom.development.js:965)
at Array.forEach ()
at forEachAccumulated (react-dom.development.js:933)
at processEventQueue (react-dom.development.js:1105)
at runEventQueueInBatch (react-dom.development.js:3600)
at handleTopLevel (react-dom.development.js:3609)
at handleTopLevelImpl (react-dom.development.js:3340)
at batchedUpdates (react-dom.development.js:11066)
at batchedUpdates (react-dom.development.js:2323)
at dispatchEvent (react-dom.development.js:3414)
```

Although I see that it's implemented: https://github.com/gabrielbull/react-desktop/blob/master/src/textInput/macOs/textInput.js#L146

Start with src/textInput/macOs/textInput.js at the focus implementation referenced in the issue, then reproduce the failure with the minimal create-react-app example. Done means the TextInput ref exposes a working focus method when the button is clicked without the TypeError.

Written by the indexing model from the issue text.