[sirinsidiator]

When you see the error next time, try to run the following script in an addon like ZAM Notebook:

for type, inventory in pairs(PLAYER_INVENTORY.inventories) do
    if(type ~= 2) then
        for k, v in ipairs(inventory.listView.data) do
            if not v.bottom then
                df("%d, %d: %s, %s: %s", type, k, tostring(v.data.bagId), tostring(v.data.slotIndex), GetItemLink(v.data.bagId, v.data.slotIndex))
            end
        end
    end
end

The chat output might give some insight into why this happens