fix: Settings FC when WiFi tethering is active.

* When filling the 'connected devices' list the function in charge of this
  assumes that the activity for which a 'new preference' is being alloc-
  cated for still exists.
* This is sometimes not the case when an user decides to navigate back to
  the 'Tethering & portable hotspot' settings menu.
* In this case the function will try to allocate a new preference for an
  activity == NULL and lead to a Settings FC.

  Logcat showing the FC: https://gist.github.com/myzb/765d03172078a7657fb0

Change-Id: I34dcafe9286443220867399f22dccb75a4b40d38