indexoutof range Exception was unhandled in c# code

private void getmaxScript()
        {

            string ConnectionStringIntellectServerDetails = "Data Source=" + txtServerNameIPIntellect.Text + ";Initial Catalog=" + lblSqlDatabase.Text + ";user id=" + txtUsernameIntellect.Text + ";password=" + txtPasswordIntellect.Text + "";
            SqlConnection conn = new SqlConnection(ConnectionStringIntellectServerDetails);
            conn.Open();

            SqlCommand cmmd = new SqlCommand("", conn);
            cmmd.CommandText = "CREATE TABLE tbl_Script([tblScript_Id] INTEGER CONSTRAINT PKeytblScript_Id Primary Key, [ScriptId] INTEGER)";
            if (conn.State == ConnectionState.Open)
            {
                try
                {
                    cmmd.ExecuteNonQuery();
                    MessageBox.Show("Add!");
                    var ScriptId = 0;
                    string sql = "Select Max(ScriptId) from tbl_Script";
                     SqlCommand cmd = new SqlCommand(sql, conn);
                     using (var reader = cmd.ExecuteReader())
                     {
                         if (reader["ScriptId"] == System.DBNull.Value)
                         {
                             ScriptId = 1;
                         }
                         else
                         {
                            ScriptId= ScriptId++;
                         }

                         globalScriptid = ScriptId.ToString();
                         string sqlmax = "insert into tbl_Script(ScriptId) values(" + globalScriptid+ ")";
                         SqlCommand cmd1 = new SqlCommand(sqlmax, conn);
                         cmd1.ExecuteNonQuery();
                     }
                }
                catch (SqlException expe)
                {
                    MessageBox.Show(expe.Message);
                    conn.Close();
                }      

            }
            else
            {
                MessageBox.Show("Error!");
            }
        }