Инициализатор типа "" выдал исключение. Не могу найти причину проблемы

public partial class Form1 : Form
    {
        DbConnector db;
        public Form1()
        {
            InitializeComponent();
            db = new DbConnector();

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Windows.Forms;
 
namespace SLRDbConnector
{
  public class DbConnector
  {
    private static string _ConnectionString = ConfigurationManager.ConnectionStrings["ConnString"].ConnectionString;
    private SqlCommand cmd_;
    private SqlConnection conn_;
    private SqlDataAdapter adptr_;
    private SqlDataReader reader_;
    private DataTable dtable_;
 
    public string getmessage { get; set; }
 
    public DbConnector()
    {
      this.conn_ = new SqlConnection(DbConnector._ConnectionString);
      this.cmd_ = new SqlCommand();
      this.dtable_ = new DataTable();
      this.adptr_ = new SqlDataAdapter();
    }
 
    public bool connect()
    {
      try
      {
        this.conn_.Open();
        this.getmessage = "successfully connected";
        return true;
      }
      catch (Exception ex)
      {
        this.getmessage = "connection error" + ex.Message;
        return false;
      }
    }
 
    public bool disconnect()
    {
      try
      {
        this.conn_.Close();
        return true;
      }
      catch (Exception ex)
      {
        return false;
      }
    }
 
    public string getSingleValue(string query, out string columnData, int index)
    {
      string str = (string) null;
      try
      {
        this.cmd_.Connection = this.conn_;
        this.cmd_.CommandText = query;
        this.connect();
        this.reader_ = this.cmd_.ExecuteReader();
        while (this.reader_.Read())
          str = this.reader_[index].ToString();
        this.getmessage = "successfull get value";
      }
      catch (Exception ex)
      {
        this.getmessage = "error" + ex.Message;
      }
      finally
      {
        this.disconnect();
      }
      columnData = str;
      return str;
    }
 
    public string performCRUD(string query)
    {
      string str = "";
      string lower = query.ToLower();
      try
      {
        this.cmd_.CommandText = query;
        this.cmd_.Connection = this.conn_;
        this.connect();
        this.cmd_.ExecuteNonQuery();
        if (lower.ToLower().Contains("insert into"))
          str = this.getmessage = " inseteted successfully ";
        else if (lower.Contains("delete form"))
          str = this.getmessage = "delete successfull";
        else if (lower.Contains("Update") && lower.Contains("set"))
          str = this.getmessage = "update successfull";
        else if (lower.Contains("Creat table"))
          str = this.getmessage = "create table successful";
      }
      catch (Exception ex)
      {
        str = this.getmessage = "failed to execute" + query + "\n resoin :" + ex.Message;
      }
      finally
      {
        this.disconnect();
      }
      return str;
    }
 
    public string fillDataGridView(string query, DataGridView dgv)
    {
      this.dtable_ = new DataTable();
      string str;
      try
      {
        this.cmd_.Connection = this.conn_;
        this.cmd_.CommandText = query;
        this.connect();
        this.adptr_.SelectCommand = this.cmd_;
        this.adptr_.Fill(this.dtable_);
        dgv.DataSource = (object) this.dtable_;
        dgv.Refresh();
        str = "Code Executed Successfully (filldatagridView()=> datalayer.cs)";
      }
      catch (Exception ex)
      {
        str = "Failed (filldatagridView()=> datalayer.cs) : " + ex.Message;
      }
      finally
      {
        this.disconnect();
        this.dtable_ = (DataTable) null;
      }
      return str;
    }
 
    public string GetSingleValueByArray(string query, out List<string> columndata, int index)
    {
      List<string> stringList = new List<string>();
      string str;
      try
      {
        this.cmd_.Connection = this.conn_;
        this.cmd_.CommandText = query.ToLower();
        this.connect();
        this.reader_ = this.cmd_.ExecuteReader();
        while (this.reader_.Read())
          stringList.Add(this.reader_[index].ToString());
        str = "Operation Successfull!";
        this.getmessage = "values successfully got from getSingleValueAsArrayByindex() function";
      }
      catch (Exception ex)
      {
        str = "Error in datalayer -> getSingleValueAsArrayByIndex() Reason:" + ex.Message;
        this.getmessage = "Error in datalayer getSingleValueAsArrayByIndex() for reader_ \n" + ex.Message;
        stringList.Clear();
      }
      finally
      {
        this.disconnect();
      }
      columndata = stringList;
      return str;
    }
 
    public void FillCombobox(string qry, ComboBox cmd)
    {
      int num = 0;
      List<string> columndata = new List<string>();
      this.GetSingleValueByArray(qry, out columndata, 0);
      foreach (string str in columndata)
      {
        if (str.Length > 0)
        {
          cmd.Items.Add((object) str);
          ++num;
        }
      }
      if (num <= 0)
        return;
      cmd.SelectedIndex = 0;
    }
 
    public string[] getArray(string query, int length)
    {
      string[] strArray = new string[length];
      try
      {
        this.cmd_.Connection = this.conn_;
        this.cmd_.CommandText = query;
        this.connect();
        this.reader_ = this.cmd_.ExecuteReader();
        while (this.reader_.Read())
        {
          for (int ordinal = 0; ordinal <= this.reader_.FieldCount; ++ordinal)
            strArray[ordinal] = this.reader_[ordinal].ToString();
        }
      }
      catch (Exception ex)
      {
      }
      finally
      {
        this.disconnect();
      }
      return strArray;
    }
  }

ConfigurationManager.ConnectionStrings["ConnString"].ConnectionString