Fix NPE if system ringtone name lookup returns null.

app/src/main/java/org/thoughtcrime/securesms/components/settings/app/notifications/NotificationsSettingsFragment.kt

@@ -234,7 +234,7 @@ class NotificationsSettingsFragment : DSLSettingsFragment(R.string.preferences__
     } else {
       val tone = RingtoneUtil.getRingtone(requireContext(), uri)
       if (tone != null) {
-        tone.getTitle(requireContext())
+        tone.getTitle(requireContext()) ?: getString(R.string.NotificationsSettingsFragment__unknown_ringtone)
       } else {
         getString(R.string.preferences__default)
       }

app/src/main/res/values/strings.xml

@@ -3538,6 +3538,7 @@
     <string name="StickerSearchDialogFragment_search_stickers">Search stickers</string>
     <string name="StickerSearchDialogFragment_no_results_found">No results found</string>
     <string name="EmojiSearchFragment__no_results_found">No results found</string>
+    <string name="NotificationsSettingsFragment__unknown_ringtone">Unknown ringtone</string>
 
     <!-- EOF -->