How do you mock a dependency using nSubstitute in abp framework,

In Abp framework, I'm trying to write a test for the FamilyAppService class below. I need to mock an IAutho instance in the constructor of the FamilyAppService. I tried mocking the IAutho then adding it to a new instance of FamilyAppService (instead of using GetRequiredService()) but I get a 'System.ArgumentNullException' error with the ObjectMapper.

FamilyAppService Class

     public class FamilyAppService : KmAppService, IFamilyAppService { 
        private readonly IAutho autho;

        public FamilyAppService( 
            IAutho autho) {
 
            this.autho = autho;
        } 

        public virtual async Task SendRequest(SendRequestInput input) {
             var family = ObjectMapper.Map<FamilyDto, Family>(f);
             // code ...
             await autho.FindUserIdByEmail(family.Email); 
        }
    }

Autho Class. I need to substitute the IAutho dependence with a Mocked class using nSubstitute

public class Autho : IAutho, ITransientDependency { 

        public Autho( ) {
           
        } 

        public virtual async Task<User> FindUserIdByEmail(string input) { 
            // Do want this code touched in test 
            // Code ...
        }
 
    }

My current test...

  [Fact]
        public async Task ShouldSendEmail() {
  
            var autho = Substitute.For<IAutho>();
            autho.FindUserIdByEmail(Arg.Any<string>()).Returns((User)null);  
    
            var familyAppService = new FamilyAppService(autho);
            // var familyAppService = GetRequiredService<IFamilyAppService>();

            // setup input code here..
            // get ObjectMapper error here
            await familyAppService.SendRequest(input);
 
            // Assert  
        }