[Settings] Avoid non-owner from crash mobile data settings

Some instance not been initialized under non-owner user mode.

Bug: 146562321
Test: manual
Change-Id: I4f8ac955754cbd8692727a1d0adf283ef3ff512c

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

@@ -118,6 +118,9 @@ public class MobileNetworkActivity extends SettingsBaseActivity
     @Override
     protected void onDestroy() {
         super.onDestroy();
+        if (mProxySubscriptionMgr == null) {
+            return;
+        }
         mProxySubscriptionMgr.removeActiveSubscriptionsListener(this);
     }