common api: handle replying to orphaned threads

lib/pleroma/web/common_api/common_api.ex

@@ -61,9 +61,14 @@ def get_visibility(%{"visibility" => visibility})
       do: visibility
 
   def get_visibility(%{"in_reply_to_status_id" => status_id}) when not is_nil(status_id) do
-    inReplyTo = get_replied_to_activity(status_id)
+    case get_replied_to_activity(status_id) do
+      nil ->
+        "public"
+
+      inReplyTo ->
         Pleroma.Web.MastodonAPI.StatusView.get_visibility(inReplyTo.data["object"])
     end
+  end
 
   def get_visibility(_), do: "public"