title: Firebase Dynamic Link always shows preview page when app is not installed
body: <p>I have an React Native app with dynamic links. The dynamic link opens correctly when the app is installed. But when the app is not installed I would like to open a specific website and not the AppStore preview page.
  Regarding to the react-native-firebase docs the "setFallbackUrl" method on iOS should handle this. But it's not working at all...</p>
  
  <p>I'm using react-native-firebase 5.2.1</p>
  
  <p>Here is my code for creating the dynamic link:</p>
  
  <pre><code>const link = new firebase.links.DynamicLink(`fallback-url&amp;para=?${someparams}`, 'my.test.link')
          .android.setPackageName('packagename')
          .ios.setBundleId('packagename')
          .ios.setFallbackUrl('fallback-url')
          .ios.setCustomScheme('customscheme')
          .android.setFallbackUrl('fallback-url')
          .analytics.setCampaign('title')
  
      firebase.links()
          .createShortDynamicLink(link, 'UNGUESSABLE')
          .then((url) =&gt; {
              this.setState({ link: url })
          });
  </code></pre>
  
  <p>Is something missing in my code or is there a specific rule like that the fallback url should be https... or whatever?</p>
  
  <p>Thanks for your help!</p>