Avoid NPE when missing network stats.

Bug: 5750339
Change-Id: I4b739dbd076b270a5a2b2e2a1e0f6e445d8c124e
diff --git a/services/java/com/android/server/net/NetworkStatsService.java b/services/java/com/android/server/net/NetworkStatsService.java
index 871ed68..f660520 100644
--- a/services/java/com/android/server/net/NetworkStatsService.java
+++ b/services/java/com/android/server/net/NetworkStatsService.java
@@ -1060,8 +1060,10 @@
         }
 
         // clear UID from current stats snapshot
-        mLastPollUidSnapshot = mLastPollUidSnapshot.withoutUid(uid);
-        mLastPollNetworkXtSnapshot = computeNetworkXtSnapshotFromUid(mLastPollUidSnapshot);
+        if (mLastPollUidSnapshot != null) {
+            mLastPollUidSnapshot = mLastPollUidSnapshot.withoutUid(uid);
+            mLastPollNetworkXtSnapshot = computeNetworkXtSnapshotFromUid(mLastPollUidSnapshot);
+        }
 
         // clear kernel stats associated with UID
         resetKernelUidStats(uid);