Merge branch '714-asynchronous-context-leaks' into 'master'

Fixed asynchronous Activity leaks in Fragments

If a Fragment has been detached its `getActivity()` method will return null, providing numerous crash possibilities within the app.

My approach to fixing this is to make Fragments use their own `runOnUiThreadUnlessDestroyed` method, which also checks if the Fragment has been detached before running the Runnable

Closes #714

See merge request !387