Update 80-90.md

Author: programmaticsoftware

aspnetcore/migration/80-90.md

@@ -72,6 +72,8 @@ In the project file, update each [`Microsoft.AspNetCore.*`](https://www.nuget.or
 
 ## Replace `UseStaticFiles` with `MapStaticAssets`
 
+Warning: It's recommended not to remove ```app.UseStaticFiles();``` but add ```app.MapStaticAssets();``` after it. Because it will cause an error if you load static files without the Assets feature.
+
 Optimize the handling of static files in your web apps by replacing <xref:Microsoft.AspNetCore.Builder.StaticFileExtensions.UseStaticFiles%2A> with <xref:Microsoft.AspNetCore.Builder.StaticAssetsEndpointRouteBuilderExtensions.MapStaticAssets%2A> in the app's `Program` file:
 
 ```diff