[Settings] Add null protection to avoid the crash

Change-Id: Id933a33ecf2931445a57cf6588a9c9123c170707
Test: manual
Bug: 202162176

src/com/android/settings/network/telephony/NetworkProviderWifiCallingGroup.java

@@ -285,7 +285,9 @@ public class NetworkProviderWifiCallingGroup extends
         }
 
         public void unregister() {
-            mTelephonyManager.unregisterTelephonyCallback(this);
+            if (mTelephonyManager != null) {
+                mTelephonyManager.unregisterTelephonyCallback(this);
+            }
         }
     }