Yeah actually the error is in the Loginform i havent this error before i created the login form..
here is all the code of LoginForm

Code:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.SqlClient;
using System.Windows.Forms;
using System.Data.Entity;
using System.Data.Entity.SqlServer;



namespace DG.DentneD
{



    public partial class LoginForm : Form
    {
        public LoginForm()
        {
            InitializeComponent();
        }

        private void textBox2_TextChanged(object sender, EventArgs e)
        {

        }

        private void LoginForm_Load(object sender, EventArgs e)
        {
            label4.Text = DateTime.Now.ToString();
        }

        private void label3_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void button1_Click(object sender, EventArgs e)


        {


            string srcConnString = null;
            var csBuilder = new SqlConnectionStringBuilder(srcConnString);
            csBuilder.Remove("Integrated Security");
            csBuilder.UserID = "sa";
            csBuilder.Password = "sasa";
            var connString = csBuilder.ToString();
            SqlConnection sqlcon = new SqlConnection("data source=DESKTOP-Q8526KR;initial catalog=dentned;integrated security=True;user=sa;password=sasa;MultipleActiveResultSets=True;App=EntityFramework"");
            string query = "Select * from dbo.doctors Where doctors_username = '" + txtUsername.Text.Trim() + "' and doctors_password = '" + txtPassword.Text.Trim() + "'";
            SqlDataAdapter sda = new SqlDataAdapter(query, sqlcon);
           

            DataTable dbdoc = new DataTable();
            sda.Fill(dbdoc);
            
            if (dbdoc.Rows.Count == 1) 

            {
                FormMain objfrmMain = new FormMain();
                this.Hide();
                if(dbdoc.Rows[0][8].ToString()!="admin")
                   
                {
                    objfrmMain.MainMenuStrip.Items[0].Visible = false;
                    objfrmMain.MainMenuStrip.Items[2].Visible = false;
                    objfrmMain.MainMenuStrip.Items[3].Visible = false;
                    objfrmMain.MainMenuStrip.Items[4].Visible = false;
                    objfrmMain.MainMenuStrip.Items[5].Visible = false;
                    objfrmMain.MainMenuStrip.Items[8].Visible = false;
                    

                }
                objfrmMain.Show();

            }
            else
            {
                MessageBox.Show("Wrong username or password !!!!!!!!!!!!!!!!!!!");
            }

            
                     
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void label2_Click(object sender, EventArgs e)
        {

        }

        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void label4_Click(object sender, EventArgs e)
        {
            label4.Text = DateTime.Now.ToString();
        }

        private void label5_Click(object sender, EventArgs e)
        {

        }
    }
}