[Garkin]

In my opinion this is a bug on ZOS side which was introduced in patch 1.5.6 because of changes in crafting UI.

Unfortunately this error occurs in local function, so it will be a bit difficult to fix by addon.

I'd probably try to redefine GetMaterialQuantity function, something like:

function ZO_SharedSmithingCreation:GetMaterialQuantity(patternIndex, materialIndex)
    local quantity
    if self.selectedMaterialCountCache[patternIndex] then
        quantity = self.selectedMaterialCountCache[patternIndex][materialIndex]
    end
    if quantity == nil then
        local _, _, stack = GetSmithingPatternMaterialItemInfo(patternIndex, materialIndex)
        self:SetMaterialQuantity(patternIndex, materialIndex, stack)
        quantity = stack
    end
    return quantity or 0
end

Or if you want to redefine just instance of this class, you are looking for function:

SMITHING.creationPanel:GetMaterialQuantity(patternIndex, materialIndex)