How to do a Knapsack application ? C#

I am doing a Knapsack application in C#.

I have an excel file where I have a list of products, price, quantity and from which I populate my datagridview1. When it is populated, I have a TextBox where I choose any amount, and then according to this amount the datagridview2 is populated with products where the total is equal to the amount that I have entered.

The problem is that I'm getting this error when I want my datagridview2 to be populated:

Unable to cast object of type 'System.DBNull' to other types....

Here is my code:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        configureDataGridViewColumns();
    }

    public bool IsReusable
    {
        get { return true; }
    }

    public void ProcessRequest(DataGridView context)
    {
        List<Item> items = new List<Item>();
        foreach (DataGridViewRow dr in dataGridView2.Rows)
        {
            //I've assumed imaginary properties ColName and ColValue in MyItem class
            items.Add(new Item(Convert.ToString(dr.Cells[1].Value), 
                Convert.ToInt32(dr.Cells[3].Value), Convert.ToInt32(dr.Cells[2].Value), 
                Convert.ToDouble(dr.Cells[0].Value)));
        }

        int capacity = Convert.ToInt32(montant.Text);

        ItemCollection[] ic = new ItemCollection[capacity + 1];

        for (int i = 0; i <= capacity; i++) ic[i] = new ItemCollection();

        for (int i = 0; i < items.Count; i++)
            for (int j = capacity; j >= 0; j--)
                if (j >= items[i].Weight)
                {
                    double quantity = Math.Min(items[i].Quantity, j / items[i].Weight);
                    for (int k = 1; k <= quantity; k++)
                    {
                        ItemCollection lighterCollection = ic[j - k * items[i].Weight];
                        int testValue = lighterCollection.TotalValue + 
                            k * items[i].Value;

                        if (testValue > ic[j].TotalValue) 
                            (ic[j] = lighterCollection.Copy()).AddItem(items[i], k);
                    }
                }

        foreach (KeyValuePair<string, int> kvp in ic[capacity].Contents)
        {
            string[] row = new string[] {kvp.Key, kvp.Value.ToString()};
            context.Rows.Add(row);
        }
    }

    private void configureDataGridViewColumns()
    {
        DataGridViewColumnCollection columns = dataGridView1.Columns;
        DataGridViewTextBoxColumn textColumn = new DataGridViewTextBoxColumn();

        textColumn.DataPropertyName = "Designation";
        textColumn.Name = "Designation";
        textColumn.HeaderText = "Designation";
        textColumn.ValueType = typeof(char);
        textColumn.Width = 180;
        columns.Add(textColumn);

        textColumn = new DataGridViewTextBoxColumn();
        textColumn.DataPropertyName = "Quantite";
        textColumn.Name = "Quantite";
        textColumn.HeaderText = "Quantite";
        textColumn.ValueType = typeof(decimal);
        textColumn.ReadOnly = true;
        textColumn.DefaultCellStyle.Format = "N2";
        textColumn.DefaultCellStyle.Alignment = DataGridViewContentAlignment.MiddleRight;
        columns.Add(textColumn);

        textColumn = new DataGridViewTextBoxColumn();
        textColumn.DataPropertyName = "Prix Unitaire";
        textColumn.Name = "Prix Unitaire";
        textColumn.HeaderText = "Prix Unitaire (CFA.)";
        textColumn.ValueType = typeof(decimal);
        textColumn.ReadOnly = true;
        textColumn.DefaultCellStyle.Format = "N2";
        textColumn.DefaultCellStyle.Alignment = DataGridViewContentAlignment.MiddleRight;
        columns.Add(textColumn);

        textColumn = new DataGridViewTextBoxColumn();
        textColumn.DataPropertyName = "Montant";
        textColumn.Name = "Montant";
        textColumn.HeaderText = "Montant (CFA.)";
        textColumn.ValueType = typeof(decimal);
        textColumn.ReadOnly = true;
        textColumn.DefaultCellStyle.Format = "N2";
        textColumn.DefaultCellStyle.Alignment = DataGridViewContentAlignment.MiddleRight;
        columns.Add(textColumn);

        Padding padding = dataGridView1.DefaultCellStyle.Padding;
        padding.Right = 10;
        dataGridView1.DefaultCellStyle.Padding = padding;
    }

    private class Item
    {
        public string Description;
        public int Weight;
        public int Value;
        public double Quantity;

        public Item(string description, int weight, int value, double quantity)
        {
            Description = description;
            Weight = weight;
            Value = value;
            Quantity = quantity;
        }
    }

    private class ItemCollection
    {
        public Dictionary<string, int> Contents = new Dictionary<string, int>();
        public int TotalValue;
        public int TotalWeight;

        public void AddItem(Item item, int quantity)
        {
            var a = ";";

            if (Contents.ContainsKey(item.Description))
            {
                Contents[item.Description] += quantity;
                Contents[item.Description] += item.Weight;
            }
            else
            {
                Contents[item.Description] = item.Weight;
            }

            Contents[item.Description] = Convert.ToInt32(string.Format("{0:D2}{1}", 
                item.Weight, quantity));

            TotalValue += quantity * item.Value;
            TotalWeight += quantity * item.Weight;
        }

        public ItemCollection Copy()
        {
            var ic = new ItemCollection();
            ic.Contents = new Dictionary<string, int>(this.Contents);
            ic.TotalValue = this.TotalValue;
            ic.TotalWeight = this.TotalWeight;
            return ic;
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        ProcessRequest(dataGridView1);
    }

    private void button2_Click(object sender, EventArgs e)
    {
        openFileDialog1.ShowDialog();
        string nomFichier = openFileDialog1.FileName;

        //string nomFichier = My.Computer.FileSystem.CurrentDirectory + "\\ex47.xlsx";
        DataSet dat = new DataSet();

        //déclaration et utilisation d'un OLeDBConnection 
        using (OleDbConnection Conn =
            new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;"
                                + " Data Source='" + nomFichier + "';"
                                + " Extended Properties=\"Excel 8.0;;HDR=YES\""))
        {
            Conn.Open();

            //déclaration du DataAdapter 
            //notre requête sélectionne toute les cellule de la Feuil1 
            using (OleDbDataAdapter Adap = new OleDbDataAdapter(
                "select * from [Feuil1$]", Conn))
            {
                Adap.TableMappings.Add("Table", "TestTable");
                //Chargement du Dataset 
                Adap.Fill(dat);
                //On charge les données sur le DGV 
                dataGridView2.DataSource = dat.Tables[0];
            }

            //libèrer les ressources 
            Conn.Close();
        }
    }
}

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The error is coming from

items.Add(new Item(Convert.ToString(dr.Cells[1].Value), Convert.ToInt32(dr.Cells[3].Value), 
    Convert.ToInt32(dr.Cells[2].Value), Convert.ToDouble(dr.Cells[0].Value)));

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What do you suggest ? Is it when I import from excel file that the error occurs in the datagridview  ?