NPE in Stk app.

Added null checks.

Bug: 35032559
Test: No tests.
Change-Id: I66e68b9492d99bbe9e19846cb5d5415ddbb6a3c7
diff --git a/src/com/android/stk/StkAppService.java b/src/com/android/stk/StkAppService.java
index c8c8e38..9eec59e 100755
--- a/src/com/android/stk/StkAppService.java
+++ b/src/com/android/stk/StkAppService.java
@@ -798,14 +798,17 @@
 
     // returns true if any Stk related activity already has focus on the screen
     private boolean isTopOfStack() {
-        ActivityManager mAcivityManager = (ActivityManager) mContext
+        ActivityManager mActivityManager = (ActivityManager) mContext
                 .getSystemService(ACTIVITY_SERVICE);
-        String currentPackageName = mAcivityManager.getRunningTasks(1).get(0).topActivity
-                .getPackageName();
+        String currentPackageName = null;
+        List<RunningTaskInfo> tasks = mActivityManager.getRunningTasks(1);
+        if (tasks == null || tasks.get(0).topActivity == null) {
+            return false;
+        }
+        currentPackageName = tasks.get(0).topActivity.getPackageName();
         if (null != currentPackageName) {
             return currentPackageName.equals(PACKAGE_NAME);
         }
-
         return false;
     }