Blazor - Copying Razor Page Causes Error 'System.NullReferenceException' in System.Private.CoreLib.dll

I have a brand new .Net6 Blazor App, rendered side, with a newly added page at /pages/scorer.razor which runs and debugs without issue.

When I copy the .razor page to a new page, eg from /pages/scorer.razor to /pages/mudscorer.razor, the following error is thrown when attempting to debug the application:

Exception thrown: 'System.NullReferenceException' in System.Private.CoreLib.dll

Source: /pages/scorer.razor

@page "/Scorer"
@attribute [Authorize()]
@attribute [Authorize(Roles = "Scoring")]
<PageTitle>MudScorer</PageTitle>

<link href="css/SleepCareStyle.css" type="text/css" rel="stylesheet" />
<style type="text/css">
    #dimScreen {
        background: rgba(255, 255, 255, 0.5);
        height: 100%;
        left: 0;
        margin: 0;
        padding: 0;
        position: fixed;
        top: 0;
        width: 100%;
    }
</style>

<table border="0" width="100%">
    <tr>
        <td class="SmallHeader">Scorer's Work List</td>
    </tr>
    <tr>
        <td>
            <ScorerWorkList></ScorerWorkList>
        </td>
    </tr>
</table>

Source: /pages/mudscorer.razor

@page "/Scorer"
@attribute [Authorize()]
@attribute [Authorize(Roles = "Scoring")]
<PageTitle>MudScorer</PageTitle>

<link href="css/SleepCareStyle.css" type="text/css" rel="stylesheet" />
<style type="text/css">
    #dimScreen {
        background: rgba(255, 255, 255, 0.5);
        height: 100%;
        left: 0;
        margin: 0;
        padding: 0;
        position: fixed;
        top: 0;
        width: 100%;
    }
</style>

<table border="0" width="100%">
    <tr>
        <td class="SmallHeader">Scorer's Work List</td>
    </tr>
    <tr>
        <td>
            <ScorerWorkList></ScorerWorkList>
        </td>
    </tr>
</table>

Make the @page directive unique otherwise the router won't know how to route requests for /Scorer

Source: /pages/mudscorer.razor

@page "/MudScorer" 
        ^^^
    for example

Incidentally, not sure why you didn't get a bit more help out of the subsystem. When I make duplicated @page directives I see:

_{It's one of my favorite runtime errors to cause these days - that and adding some new thing, forgetting to register it in DI and getting Cannot provide a value for property 'X' on type 'Y'. There is no registered service of type 'Z'. Classic copy pasta mistake, every time!}