Line-30's onPress() produces error: undefined is not an object (evaluating '_this2.drawer.openDrawer')

Drawer.jsx

import React, { Component } from 'react'
import { View, Text, TouchableOpacity, StyleSheet } from 'react-native'

import DrawerLayout from 'react-native-drawer-layout'

export default class Drawer extends Component {

  constructor(props) {
    super(props)
    this.state = {}
  }

  render() {
  
    let navigationView = (
      <View style={{ flex: 1, backgroundColor: '#fff' }}>
        <Text style={{ margin: 10, fontSize: 15, textAlign: 'left' }}>
          I'm in the Drawer
        </Text>
      </View>
    )
    
    return (
      <DrawerLayout
        drawerWidth={300}
        drawerPosition={DrawerLayout.positions.Left}
        renderNavigationView={() => navigationView}
      >
        <View style={{ flex: 1, alignItems: 'center' }}>
          <TouchableOpacity
            onPress={() => this.drawer.openDrawer()}
            style={{ backgroundColor: 'magenta' }}
          >
            <Text>Press Me!</Text>
          </TouchableOpacity>
        </View>
      </DrawerLayout>
    )
    
  }
}