This is a legacy ASP.NET C# project using web forms. In a view.ascx.cs page, there is this ShowNotification method which will try to display the message string on a popup.

However when I try to launch the page, the popup does not appear and the following error is shown in the console:

Uncaught TypeError: Sys.Application.add_load(...) is not a function

Upon clicking more details:

private void ShowNotification(string message)
{
    message = Utility.RemoveSpecialCharacters(message);
    ScriptManager.RegisterStartupScript(Page, typeof(Page), "RadAlertScript", "Sys.Application.add_load(function X(){showErrorMessage('" + message + "'); Sys.Application.remove_load(X); })", true);
}