Corona SDK 中attempt to call method 'removeSelf()'（一个nil值）的错误提示

好的，我的问题是在Corona SDK中，当我尝试在一定时间后删除爆炸对象时，出现了错误，错误提示为attempt to call method 'removeSelf()'（一个nil值）。我尝试了许多不同的解决方案，但都没有起作用。我是Corona SDK的新手，所以需要帮助。以下是出现问题的代码：

    local function addExplosion(x, y)
                local explosion = display.newImage("Explosion.png", x, y)
        physics.addBody(explosion, "static")

        timer.performWithDelay(1000, function(self)
            self:removeSelf()
        end, 1)
    end

    local function onCollision(event)
        if (event.phase == "began") then
            if(event.object1.hasCollided == false and         event.object2.hasCollided == false and event.object1.y > 50) then
                local explosionSound = audio.loadSound("Explosion.ogg")
                local explosionChannel = audio.play(explosionSound)

                local collisionX = (event.object1.x + event.object2.x) * 0.5
                local collisionY = (event.object1.y + event.object2.y) * 0.5

                event.object1.hasCollided = true
                event.object2.hasCollided = true

                event.object1:removeSelf()
                event.object2:removeSelf()

                addExplosion(collisionX, collisionY)

                score = score - 5
                scoreText.text = score
            end
        end
    end

    local function addBomb()
        local startX = math.random(display.contentWidth * 0.1, display.contentWidth * 0.9)

        local bomb = display.newImage("bomb.png", startX, -10)
        physics.addBody(bomb, "dynamic")
        bomb.hasCollided = false
        Runtime:addEventListener("enterFrame", bomb)
        bomb:addEventListener("touch", bombTouched)
    end

    local function addBalloon()
        local startX = math.random(display.contentWidth * 0.1, display.contentWidth * 0.9)

        local balloon = display.newImage("red_balloon.png", startX,         display.contentHeight + 50)
        physics.addBody(balloon, "dynamic")
        balloon.gravityScale = -0.5
        balloon.enterFrame = offscreen
        balloon.hasCollided = false
        Runtime:addEventListener("enterFrame", balloon)
        Runtime:addEventListener("collision", onCollision)
    end