SystemUI: Fix possible NPE

isTaskAboveTask() results in an NPE if the caller is null.
Wrap with null checks on lauchTargetTask and
launchTargetTask.group to prevent this

Change-Id: I372ea7744c67ddd11d9731e6bee1858d4dd3318c
Reference: BugDumps 20161021-27 L#77

Author: BadDaemon

packages/SystemUI/src/com/android/systemui/recents/views/TaskStackView.java

@@ -1018,8 +1018,9 @@ public void startLaunchTaskAnimation(TaskView tv, Runnable r, boolean lockToTask
                 t.setClipViewInStack(false);
                 t.startLaunchTaskAnimation(r, true, true, lockToTask);
             } else {
-                boolean occludesLaunchTarget = launchTargetTask.group.isTaskAboveTask(t.getTask(),
-                        launchTargetTask);
+                boolean occludesLaunchTarget = launchTargetTask != null &&
+                        launchTargetTask.group != null &&
+                        launchTargetTask.group.isTaskAboveTask(t.getTask(), launchTargetTask);
                 t.startLaunchTaskAnimation(null, false, occludesLaunchTarget, lockToTask);
             }
         }