Object reference not set to an instance of an object.how to solve this error?

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();
    }