Object reference not set to an instance of an object.

string script = "alert(\"Alert!\");";
ScriptManager.RegisterStartupScript(this, GetType(), 
                      "ServerControlScript", script, true);

public string RecoveryPassword(string UserName, string UserEmail)
        {
            DataTable Dt = new DataTable();
            string PasswordSring = string.Empty;

            base.CommandString = "Select [Password] From [User] Where [Username]=@UserName and [Email]=@UserEmail;";
            base.AddParameter("@UserName", SqlDbType.NVarChar, UserName);
            base.AddParameter("@UserEmail", SqlDbType.NVarChar, UserEmail);

            Dt = base.ExecuteQuery();

            if (Dt.Rows.Count > 0)
            {
                PasswordSring = Dt.Rows[0]["Password"].ToString();
            }
            else
            {
                PasswordSring = "Not found.";
            }

            return PasswordSring;
        }