title: TypeError:undefined is not a constructor
body: <p>Getting TypeError:undefined is not a constructor.</p>
  
  <p>Here is my <strong>Code</strong>:</p>
  
  <pre><code>import React, { Component } from 'react';
  import { StyleSheet, View } from 'react-native';
  import { StackNavigator } from 'react-navigation';
  import LoginScreen from './Screens/LoginScreen';
  
  export default function App() {
     return (
       &lt;View style={styles.container}&gt;
          &lt;appStackNavigator /&gt;
       &lt;/View&gt;
        );
    }
  
    const appStackNavigator = new StackNavigator({
      Login: { Screen: LoginScreen },
  
    });
  
  
     const styles = StyleSheet.create({
       container:
       {
         backgroundColor: '#AA00FF',
         flex: 1,
         alignItems: 'center',
         justifyContent: 'center',
       },
     });
  </code></pre>
  
  <p>Here is the <strong>Error</strong> I got:</p>
  
  <pre><code>TypeError:undefined is not a constructor(evaluating'new_reactNavigation.StackNavigator({Login:{Screen:_LoginScreen.LoginScreen}})')
  </code></pre>
  
  <p>Please help to debug. </p>
  
  <p>Edit: (post was mostly code, so added some lines in support of question)</p>