i get cannot find rows 0

<asp:TemplateField>
<ItemTemplate>
<asp:LinkButton ID="LinkButtonDeleteAssignment" runat="server" OnClick="LinkButtonDeleteAssignmentClick" Text="Delete"></asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>

protected void LinkButtonDeleteAssignmentClick(object sender, EventArgs e)
       {
           int index = 0;
           LinkButton lb = (LinkButton)sender;

           GridViewRow row = (GridViewRow)lb.NamingContainer;

           if (row != null)
           {
               index = row.RowIndex;
           }


               dt.Rows[index].Delete();

           GridViewShowAssignments.DataSource = dt;
           GridViewShowAssignments.DataBind();

protected void ButtonAddAssignmentClick(object sender, EventArgs e)
       {
           {
               //Lägg till i dbn o binda till gridviewn
               using (var db = new knowitCVdbEntities())
               {
                   SPWeb theSite = SPControl.GetContextWeb(Context);
                   SPUser theUser = theSite.CurrentUser;
                   string strUserName = theUser.LoginName;

                   var theEmplAssignment = (
                                               from p
                                                   in db.EMPLOYEES
                                               where p.username == strUserName
                                               select p).FirstOrDefault();

                   _emp = theEmplAssignment;

                   if (_emp != null)
                   {
                       //Create assignment
                       var myAssignment = new EMPLOYEES_ASSIGNMENT
                                              {
                                                  assigment_id = new Random().Next(),
                                                  employee_id = _emp.employee_id,

                                                  reference_name = TextBoxReference.Text,
                                                  company_name = TextBoxCompanyName.Text,
                                                  sector = TextBoxSector.Text,
                                                  area = TextBoxArea.Text,
                                                  from_date = TextBoxFromDate.Text,
                                                  to_date = TextBoxToDate.Text,
                                                  description = TextBoxDesc.Text,
                                              };

                       //Create assignment tools
                       for (int i = 0; i < ListBoxAssignmentTools.Items.Count; i++)
                       {
                           var myTool = new EMPLOYEES_ASSIGMENT_TOOLS()
                                            {
                                                assigment_tools_id = new Random().Next(),
                                                assigment_id = myAssignment.assigment_id,
                                                employee_id = myAssignment.employee_id,

                                                tools_name = ListBoxAssignmentTools.Items[i].ToString()
                                            };

                           myAssignment.EMPLOYEES_ASSIGMENT_TOOLS.Add(myTool);
                       }

                       //Create assignment technology
                       for (int i = 0; i < ListBoxAssignmentTechnology.Items.Count; i++)
                       {
                           var myTech = new EMPLOYEES_ASSIGMENT_TECHNOLOGY()
                                            {
                                                assigment_technology_id = new Random().Next(),
                                                assigment_id = myAssignment.assigment_id,
                                                employee_id = myAssignment.employee_id,

                                                technology_name = ListBoxAssignmentTechnology.Items[i].ToString()
                                            };

                           myAssignment.EMPLOYEES_ASSIGMENT_TECHNOLOGY.Add(myTech);
                       }

                       //Add assignment
                       _emp.EMPLOYEES_ASSIGNMENT.Add(myAssignment);
                       db.SaveChanges();
                       var dt = new DataTable();
                       if (Session["DataTable"] != null)
                       {

                           dt = (DataTable) Session["DataTable"];
                       }
                           //Populate gridview
                       else
                       {
                           dt.Columns.Add("Company name");
                           dt.Columns.Add("Sector");
                           dt.Columns.Add("Area");
                           dt.Columns.Add("From");
                           dt.Columns.Add("To");
                           dt.Columns.Add("Tools");
                           dt.Columns.Add("Technology");
                           dt.Columns.Add("Description");
                           dt.Columns.Add("Reference");
                           dt.Rows.Clear();

                       }



                       DataRow dr = dt.NewRow();
                       dr["Company name"] = TextBoxCompanyName.Text;
                       dr["Sector"] = TextBoxSector.Text;
                       dr["Area"] = TextBoxArea.Text;
                       dr["From"] = TextBoxFromDate.Text;
                       dr["To"] = TextBoxToDate.Text;
                       dr["Description"] = TextBoxDesc.Text;
                       dr["Reference"] = TextBoxReference.Text;

                       string sToolsValue = string.Empty;
                       for (int i = 0; i < ListBoxAssignmentTools.Items.Count; i++)
                       {
                           sToolsValue += ", " + ListBoxAssignmentTools.Items[i];
                       }
                       dr["Tools"] = sToolsValue;

                       string sTechValue = string.Empty;
                       for (int i = 0; i < ListBoxAssignmentTechnology.Items.Count; i++)
                       {
                           sTechValue += ", " + ListBoxAssignmentTechnology.Items[i];
                       }
                       dr["Technology"] = sTechValue;

                       dt.Rows.Add(dr);
                       Session["DataTable"] = dt;

                       GridViewShowAssignments.DataSource = dt;
                       GridViewShowAssignments.DataBind();

                       TextBoxCompanyName.Text = string.Empty;
                       TextBoxArea.Text = string.Empty;
                       TextBoxSector.Text = string.Empty;
                       TextBoxFromDate.Text = string.Empty;
                       TextBoxToDate.Text = string.Empty;
                       TextBoxDesc.Text = string.Empty;
                       TextBoxReference.Text = string.Empty;
                       ListBoxAssignmentTools.Items.Clear();
                       ListBoxAssignmentTechnology.Items.Clear();
                   }
               }

           }




       }