This fix no longer needed. Download original mod from here: https://modworkshop.net/mod/25229

All credit goes to Hinaomi and their very cool and good mod Hinaomi's Rebalance - Weapons

All I did is I put some checks at the end of weaponfactorytweakdata.lua to avoid crashing. It's not best solution but it works

-- Setting concealment 0 if its nill to avoid crash
for part_id, part_data in pairs(self.parts) do
    if part_data and part_data.stats then
        -- Checking if part has stats
        local hasAnyStats = false
        for key, value in pairs(part_data.stats) do
            hasAnyStats = true
            break
        end
        
        -- If stats exist and concealment is not set, set it to 0
        if hasAnyStats and part_data.stats.concealment == nil then
            part_data.stats.concealment = 0
        end
    end
end