Fixes app crashing because of open_split_screen

Hi.

I found that promotion makes an application crash if you call the open_split_screen from an other class than the app_delegate. 

```ruby
# session.rb
open_split_screen MenuScreen.new, AppointmentsIndex.new(nav_bar: true)
```

The above code makes the application crash, because it can't find the navigation controller. If I replace `line 115` of `lib/ProMotion/screen/screen_navigation.rb`, like I did in this PR, I am able to call the open_split_screen from an other class than the app_delegate. 

I think it's very useful to call the open_split_screen from an other class, because if you've got an login screen defending your app, you should be able to open the split screen later on, when the user is verified. But I guess there're more use cases for this.

Cheers.