Fix Settings crash when setting a null ringtone am: ec3ae8bb17

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Settings/+/18400413

Change-Id: Ibfcbed93b2aeb2d5291b4fd78da3e8092b533430
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

src/com/android/settings/DefaultRingtonePreference.java

@@ -46,6 +46,11 @@ public class DefaultRingtonePreference extends RingtonePreference {
 
     @Override
     protected void onSaveRingtone(Uri ringtoneUri) {
+        if (ringtoneUri == null) {
+            setActualDefaultRingtoneUri(ringtoneUri);
+            return;
+        }
+
         String mimeType = getContext().getContentResolver().getType(ringtoneUri);
         if (mimeType == null) {
             Log.e(TAG, "onSaveRingtone for URI:" + ringtoneUri