i have done following code i got error for submit button error -( Object reference not set to an instance of an object.) line-( keycolumns[0] = dtable.Columns["formid"];)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;


namespace Validation_Project
{
    public partial class Form1 : Form
    {
               
                DataRow    drow;
                DataTable  dtable;
               String formid;
        int flag;
        
               

        public Form1()
        {
            InitializeComponent();
        }

        private void groupBox1_Enter(object sender, EventArgs e)
        {

        }

        private void btnsubmit_Click(object sender, EventArgs e)
        {
            DataColumn[] keycolumns = new DataColumn[1];
            keycolumns[0] = dtable.Columns["formid"];
            dtable.PrimaryKey = keycolumns;



            dtable = dataSet11.Tables["WCD"];
             drow = dtable.Rows.Find(formid);
             drow.BeginEdit();
        
            drow[0] = txtvalidation.Text;
           
            drow.EndEdit();
           
            txtformid.Enabled = true;
            oleDbDataAdapter1.Update(dataSet11);
            oleDbDataAdapter1.Fill(dataSet11);
            MessageBox.Show("Record Submited.......");
         
            
        }
       
        private void btnok_Click(object sender, EventArgs e)
        {
            oleDbDataAdapter1.SelectCommand.Parameters["formid"].Value = txtformid.Text;
            oleDbDataAdapter1.Fill(dataSet11);

        }

        private void btnclear_Click(object sender, EventArgs e)
        {
            dataSet11.Clear();
        }

        private void btnclose_Click(object sender, EventArgs e)
        {
            Close();
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (txtvalidation.Text == "INVALID")
            {
                groupBox2.Visible = true;
            }
            else
            {
                groupBox2.Visible = false;
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
       
        }

       
    }
}