Cannot access a disposed object. object name 'ComboBox'.

private void cbCategory_SelectedIndexChanged(object sender, EventArgs e)
        {
string cat = cbCategory.Text;

            SqlConnection con = new SqlConnection();
            con.ConnectionString = @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\New.mdf;Integrated Security=True;User Instance=True";
            con.Open();
            SqlCommand cmd = new SqlCommand("select Category From tblItem where Category='" + cat + "'", con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);
            if (dt.Rows.Count > 0)
            {
                cat1 = dt.Rows[0][0].ToString();
            }
            con.Close();
            if (cat == cat1)
            {
                //string cat = cbCategory.Text;
                SqlConnection conn = new SqlConnection();
                conn.ConnectionString = @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\New.mdf;Integrated Security=True;User Instance=True";
                conn.Open();
                SqlCommand cmd1 = new SqlCommand("select sum(quantity) from tblItem where Category='" + cat + "'", conn);
                SqlDataAdapter daaa = new SqlDataAdapter(cmd1);
                DataTable dt1 = new DataTable();
                daaa.Fill(dt1);
                if (dt1.Rows.Count > 0)
                {
                    quantit = Convert.ToInt32(dt1.Rows[0][0].ToString());
                }
                conn.Close();
                textBox1.Text = Convert.ToString(quantit);

                SqlConnection con4 = new SqlConnection();
                con4.ConnectionString = @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\New.mdf;Integrated Security=True;User Instance=True";
                con4.Open();
                SqlCommand cmd4 = new SqlCommand("select Category From tblSalesEntry where Category='" + cat + "'", con4);
                SqlDataAdapter da4 = new SqlDataAdapter(cmd4);
                DataTable dt4 = new DataTable();
                da4.Fill(dt4);
                if (dt4.Rows.Count > 0)
                {
                    cat1 = dt4.Rows[0][0].ToString();
                }
                con4.Close();
                if (cat == cat1)
                {
                    SqlConnection con1 = new SqlConnection();
                    con1.ConnectionString = @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\New.mdf;Integrated Security=True;User Instance=True";
                    con1.Open();
                    SqlCommand cmdd = new SqlCommand("select sum(Quantity) from tblSalesEntry where Category='" + cat + "'", con1);
                    SqlDataAdapter daa = new SqlDataAdapter(cmdd);
                    DataTable dtt = new DataTable();
                    daa.Fill(dtt);
                    if (dtt.Rows.Count > 0)
                    {
                        quanti = Convert.ToInt32(dtt.Rows[0][0].ToString());
                    }
                    con1.Close();
                }
                /*else
                {
                    MessageBox.Show("THIS CATEGORY IS NOT SALED");
                }*/

                int stoc = quantit - quanti;
                textBox1.Text = Convert.ToString(stoc);
                
            }
            else
            {
                MessageBox.Show("CATEGORY DOES NOT EXIST IN TABLE");
                this.Close();
                
            }
}

System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'ComboBox'.
   at System.Windows.Forms.Control.CreateHandle()
   at System.Windows.Forms.ComboBox.CreateHandle()
   at System.Windows.Forms.Control.get_Handle()
   at System.Windows.Forms.Control.SendMessage(Int32 msg, Int32 wparam, Int32 lparam)
   at System.Windows.Forms.Control.EndUpdateInternal(Boolean invalidate)
   at System.Windows.Forms.ComboBox.EndUpdate()
   at System.Windows.Forms.ComboBox.OnDataSourceChanged(EventArgs e)
   at System.Windows.Forms.ListControl.SetDataConnection(Object newDataSource, BindingMemberInfo newDisplayMember, Boolean force)
   at System.Windows.Forms.ListControl.DataSourceDisposed(Object sender, EventArgs e)
   at System.ComponentModel.Component.Dispose(Boolean disposing)
   at System.Windows.Forms.BindingSource.Dispose(Boolean disposing)
   at System.ComponentModel.Component.Dispose()
   at System.ComponentModel.Container.Dispose(Boolean disposing)
   at System.ComponentModel.Container.Dispose()
   at Bill.StockList.Dispose(Boolean disposing) in C:\Users\Ravu\Desktop\Bill\Bill\StockList.Designer.cs:line 18
   at System.ComponentModel.Component.Dispose()
   at System.Windows.Forms.Form.WmClose(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)