[Provider Model] Fix Internet preference crash

- Post on main thread for UI update

- Add NPE checking

Bug: 178670562
Test: manual test
- atest InternetPreferenceControllerTest

Change-Id: I6c37f7dc19f9f9bbc75b36415a47aa1366dc5f5e

tests/unit/src/com/android/settings/network/InternetPreferenceControllerTest.java

@@ -36,6 +36,7 @@ import android.net.NetworkScoreManager;
 import android.net.wifi.WifiManager;
 import android.os.Handler;
 import android.os.Looper;
+import android.telephony.SubscriptionManager;
 
 import androidx.lifecycle.Lifecycle;
 import androidx.preference.Preference;
@@ -123,4 +124,13 @@ public class InternetPreferenceControllerTest {
 
         assertThat(mPreference.getSummary()).isEqualTo(TEST_SUMMARY);
     }
+
+    @Test
+    public void updateCellularSummary_getNullSubscriptionInfo_shouldNotCrash() {
+        final SubscriptionManager subscriptionManager = mock(SubscriptionManager.class);
+        when(mContext.getSystemService(SubscriptionManager.class)).thenReturn(subscriptionManager);
+        when(subscriptionManager.getDefaultDataSubscriptionInfo()).thenReturn(null);
+
+        mController.updateCellularSummary();
+    }
 }