If both the ways prodce an error your addon must have something called which both functions call/change in the end.
The error says there is something NIL which shouldn't be nil.
Check the function ZO_Tracker:ForceAssist and see what variables are used as parameters or inside the line where the error occurs (436).
Try to find out in the code of esoui where the variables get filled and work your way back to the point where you have changed these variables in your code (or a reference to them like ZO_Tracker e.g.).
Source code files of PTS are not on gitter already afaik but Cairenn has posted them here:
http://tiny.cc/Calia-APIUploads
You can download it and check the file in EsoUI/Ingame/ZO_Quest/QuestTracker.lua and work your way backwards.