Navigation correct exports

src/Navigation.js

   const generator = registeredScreens[screenID];
   if (!generator) {
     console.error(`Navigation.getRegisteredScreen: ${screenID} used but not yet registered`);
-    return;
+    return undefined;
   }
   return generator();
 }
   return platformSpecific.dismissInAppNotification(params);
 }
 
+function startTabBasedApp(params) {
+  return platformSpecific.startTabBasedApp(params);
+}
+
+function startSingleScreenApp(params) {
+  return platformSpecific.startSingleScreenApp(params);
+}
+
 export default {
   registerScreen,
   getRegisteredScreen,
   dismissLightBox,
   showInAppNotification,
   dismissInAppNotification,
-  startTabBasedApp: platformSpecific.startTabBasedApp,
-  startSingleScreenApp: platformSpecific.startSingleScreenApp
+  startTabBasedApp,
+  startSingleScreenApp
 };