[Settings] Avoid from Settings crash

Avoid from crash when null pointer.

Bug: 152336444
Test: m RunSettingsRoboTests -j ROBOTEST_FILTER=MobileNetworkActivityTest
Change-Id: I77314f697d8a99d77b9f5fe0a48ddbae69634743

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

@@ -53,7 +53,8 @@ public class MobileNetworkActivity extends SettingsBaseActivity
     @VisibleForTesting
     static final int SUB_ID_NULL = Integer.MIN_VALUE;
 
-    private ProxySubscriptionManager mProxySubscriptionMgr;
+    @VisibleForTesting
+    ProxySubscriptionManager mProxySubscriptionMgr;
     private int mCurSubscriptionId;
 
     @Override
@@ -102,9 +103,7 @@ public class MobileNetworkActivity extends SettingsBaseActivity
             actionBar.setDisplayHomeAsUpEnabled(true);
         }
 
-        mProxySubscriptionMgr = ProxySubscriptionManager.getInstance(this);
+        registerActiveSubscriptionsListener();
-        mProxySubscriptionMgr.setLifecycle(getLifecycle());
-        mProxySubscriptionMgr.addActiveSubscriptionsListener(this);
 
         final Intent startIntent = getIntent();
         validate(startIntent);
@@ -119,15 +118,26 @@ public class MobileNetworkActivity extends SettingsBaseActivity
         maybeShowContactDiscoveryDialog(mCurSubscriptionId);
     }
 
+    @VisibleForTesting
+    void registerActiveSubscriptionsListener() {
+        mProxySubscriptionMgr = ProxySubscriptionManager.getInstance(this);
+        mProxySubscriptionMgr.setLifecycle(getLifecycle());
+        mProxySubscriptionMgr.addActiveSubscriptionsListener(this);
+    }
+
     /**
      * Implementation of ProxySubscriptionManager.OnActiveSubscriptionChangedListener
      */
     public void onChanged() {
         SubscriptionInfo info = getSubscription();
         int oldSubIndex = mCurSubscriptionId;
-        int subIndex = info.getSubscriptionId();
         updateSubscriptions(info);
+
         // Remove the dialog if the subscription associated with this activity changes.
+        if (info == null) {
+            return;
+        }
+        int subIndex = info.getSubscriptionId();
         if (subIndex != oldSubIndex) {
             removeContactDiscoveryDialog(oldSubIndex);
         }

tests/robotests/src/com/android/settings/network/telephony/MobileNetworkActivityTest.java

@@ -22,6 +22,7 @@ import static com.google.common.truth.Truth.assertThat;
 
 import static org.mockito.Mockito.anyInt;
 import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.mock;
 import static org.robolectric.Shadows.shadowOf;
 
 import android.content.Context;
@@ -39,6 +40,7 @@ import androidx.test.core.app.ApplicationProvider;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 
 import com.android.internal.telephony.TelephonyIntents;
+import com.android.settings.network.ProxySubscriptionManager;
 
 import org.junit.After;
 import org.junit.Before;
@@ -109,6 +111,12 @@ public class MobileNetworkActivityTest {
 
         private SubscriptionInfo mSubscriptionInFragment;
 
+        @Override
+        void registerActiveSubscriptionsListener() {
+            mProxySubscriptionMgr = mock(ProxySubscriptionManager.class);
+            onChanged();
+        }
+
         @Override
         void switchFragment(SubscriptionInfo subInfo) {
             mSubscriptionInFragment = subInfo;