Fixed a crash when dumping groupmanager

Change-Id: Id5ffb9e61c1a17d5bd7d8f58024bf84c0fbb6781
Fixes: 28706404
diff --git a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationGroupManager.java b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationGroupManager.java
index d2326d2..270b6ed 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationGroupManager.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationGroupManager.java
@@ -459,7 +459,8 @@
 
         @Override
         public String toString() {
-            String result = "    summary:\n      " + summary.notification;
+            String result = "    summary:\n      "
+                    + (summary != null ? summary.notification : "null");
             result += "\n    children size: " + children.size();
             for (NotificationData.Entry child : children) {
                 result += "\n      " + child.notification;