Add null check for CarrierConfig

CarrierConfig maybe null when config loader is not ready, so we need
to add null check here.

Bug: 124447166
Test: RunSettingsRoboTests
Change-Id: I3c104f1ca44bb0bcacad460c68084163c758746a

tests/robotests/src/com/android/settings/network/telephony/gsm/AutoSelectPreferenceControllerTest.java

@@ -111,4 +111,12 @@ public class AutoSelectPreferenceControllerTest {
                 mContext.getString(R.string.manual_mode_disallowed_summary,
                         mTelephonyManager.getSimOperatorName()));
     }
+
+    @Test
+    public void init_carrierConfigNull_shouldNotCrash() {
+        when(mCarrierConfigManager.getConfigForSubId(SUB_ID)).thenReturn(null);
+
+        // Should not crash
+        mController.init(SUB_ID);
+    }
 }