I am currently working in the Sample Charts application in Oracle APEX version 5.1.2. I would like to edit the code for the Gantt Chart plug-in, but for some reason it won't let me type in the PL/SQL code box or erase anything in the box. I read somewhere online that only the master copy of the plug-in can be customized and that the Reference Master Plug-in From box has to be blank. However, it does say that "This is the "master" copy of this plug-in" and I can't erase or choose a blank value for the Reference Master Plug-in From box. In addition, on the screen that shows the list of all the plug-ins for the application, it says that this plug in was subscribed from a "Nonexistent Master".
This is what I am seeing on my screen:


Has anyone seen this before and/or know how to fix this problem?
Thank you in advance.