Uncatchable crash when storing a string set that contains a null item

[pablobaxter]

A string set that contains a null item causes HarmonyTransaction to crash, due to it not handling null values in string sets. This only occurs when using apply due to the transactional nature of Harmony updates that allow it to do quick reloads. Example code below:

val prefs = context.getHarmonySharedPreferences("prefs")
val testHashSet = hashSetOf<String?>(null)
prefs.edit().putStringSet("test", testHashSet).apply()

This causes a crash in an internal background thread.