Tethering: Throw exception when register null callback to avoid a crash

Register null callback will cause a crash so throw an exception

Test: m, CtsTetheringTest
Change-Id: Id4337328f6793f1436c9d3561960fa69d17d030e
diff --git a/Tethering/src/com/android/networkstack/tethering/Tethering.java b/Tethering/src/com/android/networkstack/tethering/Tethering.java
index 873961a..025f805 100644
--- a/Tethering/src/com/android/networkstack/tethering/Tethering.java
+++ b/Tethering/src/com/android/networkstack/tethering/Tethering.java
@@ -2393,6 +2393,9 @@
                 hasCallingPermission(NETWORK_SETTINGS)
                         || hasCallingPermission(PERMISSION_MAINLINE_NETWORK_STACK)
                         || hasCallingPermission(NETWORK_STACK);
+        if (callback == null) {
+            throw new NullPointerException();
+        }
         mHandler.post(() -> {
             mTetheringEventCallbacks.register(callback, new CallbackCookie(hasListPermission));
             final TetheringCallbackStartedParcel parcel = new TetheringCallbackStartedParcel();