Fixing NPE when there is no running activity.

Bug: 30668904
Test: Boot the device, kill launcher
Change-Id: I10236d33c7fbc1d20b5d4b097e0660596188eaf0

packages/SystemUI/src/com/android/systemui/recents/RecentsImpl.java

@@ -374,6 +374,10 @@ public class RecentsImpl implements ActivityOptions.OnAnimationFinishedListener
         MutableBoolean isHomeStackVisible = new MutableBoolean(true);
         if (!ssp.isRecentsActivityVisible(isHomeStackVisible)) {
             ActivityManager.RunningTaskInfo runningTask = ssp.getRunningTask();
+            if (runningTask == null) {
+                return;
+            }
+
             RecentsTaskLoader loader = Recents.getTaskLoader();
             sInstanceLoadPlan = loader.createLoadPlan(mContext);
             sInstanceLoadPlan.preloadRawTasks(!isHomeStackVisible.value);