[Darkonode]

OMG and I LITERALLY noticed it straight after i posted T.T. Well, for your entertainment it was this line in the function I posted

tab.SetScript("OnClick", Tab_OnClick)

which should be

tab:SetScript("OnClick", Tab_OnClick)

Now it works! Almost... I'm still missing all the contents I wish to be shown on the tabs.

EDIT: I figured it out! I forgot a table insert from the SetTabs function, which made it return nil for the PopulateTabs function causing a lua error.