Here’s my code that declares & sets ReturnUrl.

public class ExPageBase : ExComponentBase
{
    [Parameter]
    [SupplyParameterFromQuery(Name = "returnUrl")]
    public string? ReturnUrl { get; set; }

    [MemberNotNull (nameof(ReturnUrl))]
    protected override async Task OnInitializedAsync()
    {

        await base.OnInitializedAsync();

        if (string.IsNullOrEmpty(ReturnUrl))
        {
            if (Principal.IsSysAdmin())
                ReturnUrl = GlobalConstants.UrlSysAdminHomePage;
            else if (Principal.IsManager())
                ReturnUrl = GlobalConstants.UrlManagerHomePage;
            else if (Principal.IsOnlyVolunteer())
                ReturnUrl = GlobalConstants.UrlVolunteerHomePage;
            else
                ReturnUrl = GlobalConstants.UrlHomePage;
        }
    }
}

I am getting the following warning when I compile:

5>C:GitLouisHoweLouisHowe.webSharedExPageBase.cs(51,4,51,35): warning CS8774: Member 'ReturnUrl' must have a non-null value when exiting.

Line 51 is: await base.OnInitializedAsync();

Why does it say that ReturnUrl must have a non-null value anywhere for any reason?

And why in particular when exiting a call to the base OnInitializedAsync() when I am still inside my OnInitializedAsync()?

I can understand it giving me a warning due to the MemberNotNull when exiting from my OnInitializedAsync(). But why from calling the base one?

This code is for Blazor Interactive Server although I think that is irrelevant to this question.