Update App.js

When fontsLoaded == false and isLoadingComplete == true then none of the branches is executed and the App component returns undefined in the render function which results in invariant violation: Nothing was returned from render.
This small fix should address this by returning null until fonts are loaded

App.js

@@ -75,6 +75,8 @@ export default props => {
         </GalioProvider>
       </NavigationContainer>
     );
+  } else {
+    return null
   }
 }