unregistered null callback will cause a crash later

Bug: N/A
Change-Id: I7acbc084d8a401817879d416f0283ca40e060069
Test: N/A

Tethering/src/com/android/networkstack/tethering/Tethering.java

@@ -2407,6 +2407,9 @@ public class Tethering {
 
     /** Unregister tethering event callback */
     void unregisterTetheringEventCallback(ITetheringEventCallback callback) {
+        if (callback == null) {
+            throw new NullPointerException();
+        }
         mHandler.post(() -> {
             mTetheringEventCallbacks.unregister(callback);
         });