Add a null check for the OnStartTetheringCallback.

This avoids a NullPointerException when trying to call the callback
and gives a more readable error message.

(cherry picked from commit 35e99ea8ef)

Change-Id: Ia419ff68ef10f308f9e44be47420e27099ee6070

core/java/android/net/ConnectivityManager.java

@@ -2079,6 +2079,8 @@ public class ConnectivityManager {
     @SystemApi
     public void startTethering(int type, boolean showProvisioningUi,
             final OnStartTetheringCallback callback, Handler handler) {
+        checkNotNull(callback, "OnStartTetheringCallback cannot be null.");
+
         ResultReceiver wrappedCallback = new ResultReceiver(handler) {
             @Override
             protected void onReceiveResult(int resultCode, Bundle resultData) {
@@ -2089,6 +2091,7 @@ public class ConnectivityManager {
                 }
             }
         };
+
         try {
             mService.startTethering(type, wrappedCallback, showProvisioningUi);
         } catch (RemoteException e) {