Add null check before finish SystemUI Tuner's activity

SystemUI Tuner's activity changes to null when Multi-window's state is
changed after confirmation dialog about removing SystemUI Tuner is
shown. Then NullPointerException occurs by trying to finish the
activity to tap "REMOVE" button on the confirmation dialog.
So add null check before finish the activity.

Bug: 32786029
Test: manual - remove System UI Tuner on Multi-Window mode

Change-Id: I455d17c0c600eeb05c9dfb7d609d40307de2f855