Quick Tip Project Server 2013 Online PDP

Hi
I have been working on this Project Server 2013 environment for the last week , yesterday i have had this problem that almost made me pull my hair out

simply i have a costume field that is linked to a lookup table , the custom field is a RAG so it should display graphical indicator... simply  i created  it and added the custom field to the PDP ... open the project to edit to find this

This Web Part was unable to load.
Information that may be useful in solving this problem was written to the Unified Logging Service (ULS) log 

an error in the web part with a nice GUID that to a ULS entry that i don't have access to ( i have opened a Support request with Microsoft) meanwhile i couldn't just wait around so i started trying to figure out what is the problem , i have noticed that the web-part do show normally when the project is in read only mode the problem only happen when i open the project in edit mode , so it must be a problem when project server try to read the custom field and what all of those custom fields have in common they are new and empty

to fix this i went to the lookup table added a new entry (N/A) and from the custom field i selected the new value as a default , that did solve the issue for me

hope this help
have fun