Fix potential NPE crash in ConversationHeaderPreferenceController

mChannel is nullable and we have to do a null-check before calling its method.

Bug: 245506600

Test: manual and atest
Change-Id: Ib739f0f66f1a2aee1b2741263e7c206341782892
diff --git a/src/com/android/settings/notification/app/ConversationHeaderPreferenceController.java b/src/com/android/settings/notification/app/ConversationHeaderPreferenceController.java
index f99a56a..43b50c8 100644
--- a/src/com/android/settings/notification/app/ConversationHeaderPreferenceController.java
+++ b/src/com/android/settings/notification/app/ConversationHeaderPreferenceController.java
@@ -126,8 +126,12 @@
 
     @VisibleForTesting
     CharSequence getLabel() {
-        return mConversationInfo != null
-                ? mConversationInfo.getLabel()
-                : mChannel.getName();
+        CharSequence label = null;
+        if (mConversationInfo != null) {
+            label = mConversationInfo.getLabel();
+        } else if (mChannel != null) {
+            label = mChannel.getName();
+        }
+        return label;
     }
 }
diff --git a/tests/robotests/src/com/android/settings/notification/app/ConversationHeaderPreferenceControllerTest.java b/tests/robotests/src/com/android/settings/notification/app/ConversationHeaderPreferenceControllerTest.java
index 9b166ac..9a1a3ab 100644
--- a/tests/robotests/src/com/android/settings/notification/app/ConversationHeaderPreferenceControllerTest.java
+++ b/tests/robotests/src/com/android/settings/notification/app/ConversationHeaderPreferenceControllerTest.java
@@ -20,6 +20,7 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.Mockito.mock;
@@ -121,6 +122,9 @@
         NotificationChannel channel = new NotificationChannel("cid", "cname", IMPORTANCE_NONE);
         mController.onResume(appRow, channel, null, null, null, null, null);
         assertEquals(channel.getName(), mController.getLabel());
+
+        mController.onResume(appRow, null, null, null, null, null, null);
+        assertNull(mController.getLabel());
     }
 
     @Test