getCurrentScreenObject() error on some custom screens

[mleroi]

When displaying a custom screen, ThemeApp.getCurrentScreenObject() triggers an error in some cases.
We've seen the error when displaying a login form at app launch for example.
The error seems to be due to the fact that current_view.custom_page_data can be undefined here:
https://github.com/uncatcrea/wp-appkit/blob/master/wp-appkit/app/core/theme-app.js#L981