Correctly marked Topic property as non-nullable

This was left over from a previous attempt to handle a CS8618 warning. While a `default!` value was assigned, and the `[NotNull, DisallowNull]` attributes were removed, the type continued to be marked as non-nullable. This is fixed.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Models/EditorViewModel.cs

@@ -22,7 +22,7 @@ public record EditorViewModel {
     ///   The <see cref="EditingTopicViewModel"/> representing the core properties of the currently selected <see
     ///   cref="Topic"/>.
     /// </summary>
-    public EditingTopicViewModel? Topic { get; init; } = default!;
+    public EditingTopicViewModel Topic { get; init; } = default!;
 
     /*==========================================================================================================================
     | CONTENT TYPE DESCRIPTOR