Error in BackHandler() React-Native

I am trying to handle back button into the mobile app. I am getting some error again and again whenever I try to go back into my app screens. My code is as under : -

 BackHandler.addEventListener("hardwareBackPress", () => {
  if (this.navigator && this.navigator.getCurrentRoutes().length > 1) {
    this.navigator.pop()
    return true // do not exit app
  } else {
    return false // exit app
  }
})

Note : - I'm using react native version 0.44.2.

Any help would be appriciated.

Thanks

via MaMalik