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

src/com/android/settings/notification/app/ConversationHeaderPreferenceController.java

@@ -126,8 +126,12 @@ public class ConversationHeaderPreferenceController extends NotificationPreferen
 
     @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;
     }
 }