Adding NPE check when the system has no tasks.

Bug: 20640135
Change-Id: Ifb62d18dbf2125bc33325b7c2f2a44da51309325

packages/SystemUI/src/com/android/systemui/recents/misc/SystemServicesProxy.java

@@ -227,7 +227,7 @@ public class SystemServicesProxy {
     /** Returns the top task. */
     public ActivityManager.RunningTaskInfo getTopMostTask() {
         List<ActivityManager.RunningTaskInfo> tasks = getRunningTasks(1);
-        if (!tasks.isEmpty()) {
+        if (tasks != null && !tasks.isEmpty()) {
             return tasks.get(0);
         }
         return null;