Hmm Interesting ..
Managed to get it not error out in my addon with the following minimal code:
Lua Code:
ObjectiveTrackerFrame:SetAllPoints(frame)
ObjectiveTrackerFrame:SetWidth(frame:GetWidth())
ObjectiveTrackerBlocksFrame:SetWidth(frame:GetWidth())
ObjectiveTrackerBlocksFrame:SetHeight(XObjectivesDB.ScrollHeight)
ObjectiveTrackerBlocksFrame:SetPoint("LEFT",30,0)
frame:SetScrollChild(ObjectiveTrackerBlocksFrame)
frame being a ScrollFrame parented to UIParent or with nUI Infopanel as necessary.
However, this offsets the tracker lines too far to the left and and makes the POI Icons and check marks not appear properly or at all.
The line that fixes that is :
DEFAULT_OBJECTIVE_TRACKER_MODULE.blockOffsetX = 30
But the moment you use that or any of the specific modules versions it kicks out the error.
Also, the use of these two lines after that code (although not sure if I need it now I'm not re-parenting as such) also produces the error.
ObjectiveTracker_Update()
QuestObjectiveTracker_UpdatePOIs()
The moment I remove those 3 lines from the equation it allows you to click the button.
Edit:
I wonder if creating your own module stops the error. Might have to be something to look into at some point.