My decryption show that error : Buffer cannot be null. Parameter name: buffer. Please help me to solve this problem.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Security.Cryptography;//use to run the encryption
using System.Text;// using ASCII language

public partial class About : System.Web.UI.Page
{
    string cipherData;
    byte[] cipherByte;
    byte[] textByte1;
    byte[] textByte2;
    byte[] key;
    
    

    SymmetricAlgorithm desObj;

    protected void Page_Load(object sender, EventArgs e)
    {
        desObj = Rijndael.Create();//when the page is load to let the function can run.
    }
    protected void btnEncryption_Click(object sender, EventArgs e)
    {
        cipherData = txtST.Text;
        textByte1 = Encoding.ASCII.GetBytes(cipherData);
        key = Encoding.ASCII.GetBytes("0123456789abcdef");//fix the size from oringinal data? dk want to 16bit or not
        desObj.Key = key;

        desObj.Mode = CipherMode.CBC;//can either choose one
        desObj.Padding = PaddingMode.PKCS7;//typd of encryption
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        CryptoStream cs = new CryptoStream(ms, desObj.CreateEncryptor(), CryptoStreamMode.Write);
        cs.Write(textByte1, 0, textByte1.Length);
        cs.Close();
        cipherByte = ms.ToArray();
        ms.Close();
        txtEnc.Text = Encoding.ASCII.GetString(cipherByte);
    }
    protected void btnDecryption_Click(object sender, EventArgs e)
    {

        System.IO.MemoryStream ms1 = new System.IO.MemoryStream(cipherByte);
        CryptoStream cs1 = new CryptoStream(ms1, desObj.CreateDecryptor(), CryptoStreamMode.Read);

        cs1.Read(cipherByte, 0, cipherByte.Length);
        textByte2 = ms1.ToArray();
        cs1.Close();
        ms1.Close();

        txtDec.Text = Encoding.ASCII.GetString(textByte2);

    }
}

Buffer cannot be null. Parameter name: buffer.