System.AggregateException exception while attempting to register a service in ASP.NET CORE 3.1

I have made an ISignUp interface which is implemented by the SignUp class to create a new user. I am using Dependancy Injection (DI). The issue is that I cannot register this service. Instead of creating new users in the SignUpController directly, I rather prefer calling a helper class via the Interface it implements.

ASP.Net Object is set back to null after OnGet() method

I build a shopping website and I encountered this error when I tried to post some data.The weird thing is the object apparently is null, even though, I can place the product’s name,price,photo and quantity on the page.I have a product class and a product model class which contains a list of products.In the product model class constructor I create the list with the products that I want to display on the site.Anyway, I have the “Clothing” page which has a list of “Product” objects and it uses the ProductModel class to access the product list which has all the data regarding the products that I want to display.When I used a breakpoint to check if the products are sent to the “Clothing” page, I found out that the Product list is null.