BNW does this in dropItems function - when it decides to spill something after trying to figure out what to spill:

    if count > 0 then
        -- now we're forced to spill items
        entity = entity or global.forces[player.force.name].roboport
        entity.surface.spill_item_stack(entity.position, {name = name, count = count}, false, entity.force, false)
    end

I have changed to this since above will crash if you run with mods that introduce items that are unknown or you hover cursor over those unknown items when they are on the ground or when you open a gui - like in Oarc, and hover them over the unknown items on the ground, OR when you drop a building on top of a rock - which is then an entity of ghost OR a tank running over a stone .

    if count > 0 then
        -- now we're forced to spill items
        entity = entity or global.forces[player.force.name].roboport
        if entity and entity.valid then
            if (global.enable_oe_debug) then
                log("dropItems: Spilling items for: ".. entity.name .. ", type: " .. entity.type .. ", at " .. GetGPStext(entity.position) .. ", entity force: ".. entity.force.name)
            end
            if (entity.name == "oarc-gui") then
                game.players[player.name].print("Close your menu when stealing ore from someone ;)")                    
            elseif (entity.surface == nil) then
                game.players[player.name].print("Send this log to JustGoFly - it shows something that WOULD have crashed, and provides good info to debug what caused it")
                log("dropItems: would have crashed accessing entity.surface - on entity name: " .. entity.name .. " for item: " .. name .. " count: " .. count .. " for player: " .. player.name)
            else
                entity.surface.spill_item_stack(entity.position, {name = name, count = count}, false, entity.force, false)
            end
        end
    end

The check for oarc-gui should be removed in your code, but the validation of the surface being nil is very important.

LOL - also the message about sending to JustGoFly. This is a catch all to debug the next issue that causes dropItem to crash.

Simplified for BNW this would be:

    if count > 0 then
        -- now we're forced to spill items
        entity = entity or global.forces[player.force.name].roboport
        if entity and entity.valid then                  
        if (entity.surface ~= nil) then
                entity.surface.spill_item_stack(entity.position, {name = name, count = count}, false, entity.force, false)
            end
        end
    end

Whee, I've been paying much less attention than I should. This looks interesting.