Fix NPE that netd callback call null TetheringConfiguration object

Bug: 148386056
Test: -build, flash, boot
      -atest TetheringTests

Change-Id: I3b1dd958fc107fdc9a6059fbdcf358af4822ebde
diff --git a/packages/Tethering/src/com/android/server/connectivity/tethering/Tethering.java b/packages/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
index 5b35bb6..19e4c10 100644
--- a/packages/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
+++ b/packages/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
@@ -272,13 +272,6 @@
 
         mStateReceiver = new StateReceiver();
 
-        mNetdCallback = new NetdCallback();
-        try {
-            mNetd.registerUnsolicitedEventListener(mNetdCallback);
-        } catch (RemoteException e) {
-            mLog.e("Unable to register netd UnsolicitedEventListener");
-        }
-
         final UserManager userManager = (UserManager) mContext.getSystemService(
                 Context.USER_SERVICE);
         mTetheringRestriction = new UserRestrictionActionListener(userManager, this);
@@ -287,6 +280,14 @@
 
         // Load tethering configuration.
         updateConfiguration();
+        // NetdCallback should be registered after updateConfiguration() to ensure
+        // TetheringConfiguration is created.
+        mNetdCallback = new NetdCallback();
+        try {
+            mNetd.registerUnsolicitedEventListener(mNetdCallback);
+        } catch (RemoteException e) {
+            mLog.e("Unable to register netd UnsolicitedEventListener");
+        }
 
         startStateMachineUpdaters(mHandler);
         startTrackDefaultNetwork();