title: react navigation not rendering screens
body: <p>I currently building a simple react native app which has two screens. I am using a react navigation to navigate between screens and when I tried following this guide <a href="https://reactnavigation.org/docs/en/hello-react-navigation.html" rel="nofollow noreferrer">https://reactnavigation.org/docs/en/hello-react-navigation.html</a>, It doesn't render anything, just displaying a blank screen. Maybe I'm missing something that's why it doesn't render. Here's my js files:</p>
  
  <p>App.js</p>
  
  <pre class="lang-js prettyprint-override"><code>import React, { Component } from 'react';
  import { Text, View, StyleSheet } from 'react-native';
  import { createAppContainer } from 'react-navigation';
  import { createStackNavigator } from 'react-navigation-stack';
  import Login from './src/components/screens/login';
  import Order from './src/components/screens/order';
  
  const AppNavigator = createStackNavigator(
    {
      Login: Login,
      Order: Order
    },
    {
      initialRouteName: 'Login',
    }
  );
  
  export default class Sales extends Component {
    render() {
      return (
        &lt;View&gt;
          &lt;App /&gt;
        &lt;/View&gt;
      );
    }
  } 
  </code></pre>
  
  <p>login.js</p>
  
  <pre class="lang-js prettyprint-override"><code>class Login extends Components {
    static navigationOptions = {
      headerShown: false,
    };
  
    render() {
      return(
        &lt;View&gt;
          &lt;Button title="Login"
            onPress={() =&gt; this.props.navigation.navigate('Order')}
          /&gt;
        &lt;/View &gt;
      );
    }
  }
  </code></pre>
  
  <p>order.js</p>
  
  <pre class="lang-js prettyprint-override"><code>class Order extends Component {
    static navigationOptions = {
      headerShown: false,
    };
    render() {
      return (
        &lt;View&gt;
          &lt;View style={styles.button}&gt;
            &lt;Button 
              title="Create Order"
              color='#65639E'
            /&gt;
          &lt;/View&gt;
        &lt;/View&gt;
      );
    }
  }
  </code></pre>