title: Cannot read property 'navigate' of Undefined in React Navigation
body: <p>i'm React Native newbie. What i'm trying to do is added <a href="https://facebook.github.io/react-native/docs/navigation.html" rel="nofollow noreferrer">react navigation</a> to my login page where user can click a button and navigate to the sign up page but i'm getting an error <code>Cannot read property 'navigate' of Undefined</code>. I've already searched the solution over an internet but no luck. This So does not help me - <a href="https://stackoverflow.com/questions/42793844/react-navigation-cannot-read-property-navigate-of-undefined">React Navigation - cannot read property &#39;navigate&#39; of undefined</a> and same with others .</p>
  
  <p>Here is my code</p>
  
  <p><strong>index.ios.js</strong></p>
  
  <pre><code>import React, { Component } from 'react'; 
  import {
    AppRegistry,
    StyleSheet,
    Text,
    View
  } from 'react-native';
  import {StackNavigator} from 'react-navigation';
  import Login from './src/screens/Login';
  import Signup from './src/screens/Signup';
  
  export default class tapak extends Component {
    constructor(props) {
      super(props);
      this.buttonPress = this.buttonPress.bind(this);
    }
  
    render() {
      return (
        &lt;View style={styles.container}&gt;
          &lt;Text style={{color: 'blue'}} onPress={this.buttonPress}&gt;sign up&lt;/Text&gt;
        &lt;/View&gt;
      );
    }
  
    buttonPress() {
      console.log('called');
      this.props.navigation.navigate('Signup');
    }
  }
  
  const Stacks = StackNavigator({
      Login: {
        screen: Login
      },
      Signup:{
        screen: Signup
      }
  });
  </code></pre>