In my Blazor-server-side project, i resolved this issue with following two steps.
Step 1: First create a new empty razor component named EmptyLayout.
EmptyLayout.razor
@inherits LayoutComponentBase
<div class="main">
<div class="content px-4">
@Body
</div>
</div>
Step 2, To set Layout=null, I use the below code in the top of all required pages
@layout EmptyLayout