[Borlox]

Putting the following code before the call to StaticPopup_Hide should prevent the Popup from declining the invite:

for i=1, STATICPOPUP_NUMDIALOGS do
    local dlg = _G["StaticPopup"..i]
    if dlg.which == "PARTY_INVITE" then
        dlg.inviteAccepted = 1
        break
    end
end

It loops through all StaticPopups and sets the inviteAccepted flag for the one that is showing the invite message.