wifi: fix NullPointerException when tethering is on and devices are connected

* When the Portable Wi-Fi hotspot is on, devices are connected to the
  hotspot, the list is being updated and the user decides to navigate
  back to the 'Tethering & portable hotspot' settings a NullPointer-
  Exception can occur due to getActivity() = null.
* Fix this by only updating the list if the activity exists.

  Logcat: https://gist.github.com/Skaveelicious/765d03172078a7657fb0

Change-Id: I34dcafe9286443220867399f22dccb75a4b40d38