Fix: Crash when adding with share function

Fixes [issue#3](https://github.com/ilesinge/scuttloid/issues/3)

The trick was to check if the bookmarkContent was initialized.
I realized, that the app only crashes, if it is fully closed (removed from the multitasking screen). In case that you want to reproduce the problem.

PS: I'm sorry, I don't know how to attach this to issue#3