未将对象引用设置来解决这个错误的object.how的一个实例

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未将对象引用设置到对象的实例。如何解决这个错误？

编码:

protected void btncart_Click(object sender, EventArgs e)

    {

        if (con.State == ConnectionState.Closed)

        {

            con.Open();

        }

        int Quantity = Convert.ToInt32(0);

        if (Quantity == 0)

        {

            Quantity = 1;

        }

        else

        {

            Quantity = Quantity + 1;

        }

        mas_id();

        string pro_id = (((Label)DataList1.Controls[0].FindControl("pro_id")).Text).ToString();

        string pro_name = (((Label)DataList1.Controls[0].FindControl("pro_name")).Text).ToString();

        float Price = float.Parse(((Label)DataList1.Controls[0].FindControl("Price")).Text);

        string pro_img = ((Image)DataList1.Controls[0].FindControl("pro_img")).ImageUrl;

        //int quantity = Convert.ToInt32(((TextBox)DataList1.Controls[0].FindControl("txtquantity")).Text);

        string master_id = (((Label)DataList1.Controls[0].FindControl("master_id")).Text).ToString();//error comes here

        SqlCommand cmd = new SqlCommand("insert into addcart values('" + master_id + "','" + pro_id + "','" + pro_name + "','" + Quantity + "','" + Price + "','" + pro_img + "')", con);

        cmd.ExecuteNonQuery();

        con.Close();

    }