Got this just after delivering the quest.

I'm assuming that is line 225 in QuestieTracker.lua? If so this is occurring in the QuestieTracker:createOrGetTrackingButton(index) function which doesn't get called while a quest is being completed.

Looking over lines 222 - 225 in the current build:

    btn:SetScript("OnEnter", function()
        local questHash = btn.hash;
        local quest = QuestieCachedQuests[questHash];
        local questTitle = quest["questName"];

...based completely on the error, and a little logic, it's not setting the "quest" variable:

local quest = QuestieCachedQuests[questHash];

This (lines 222 --> 225 and below it) little chunk of code displays a tool-tip when you hover over the tracker... now this error could be occur'ing when it was trying to build the tool-tip just after or during the quest was completed. If it was just a "one off" I'd just call it bad timing. But if it keeps happening over and over again or if you figure out a consistent bug reproduction steps, please let us know. Otherwise, we'll have to build in some kind of nil check if it becomes a huge problem. So far I haven't encountered this issue.

Are you running the latest release?

Yes, im running latest release with the install instruction. This is the first time i got it, not a problem at all.

I had a server rollback, so i had to do over the quest. This time i did not get the error, and the quest tracker was not active when i completed the quest.

Given the fact that it's tool-tip code that makes perfect sense. Please let us know if it happens again. Thanks for the bug report! /cheers