Due to AutoFixture and NSubstitude object reference not set to an instance of an object error from unit test

I am writing unit test in mvc project with AutoFixture and NSubstitude. But I am getting object reference error in my update and delete methods. Probably because I'm new, I may be missing the settings for these packages.

This is my main code

public async Task<ServiceResponse<ParameterDto>> Update(ParameterDto parameterDto)
        {
            try
            {
                if (parameterDto == null)
                {
                    return serviceResponseHelper.SetError<ParameterDto>(null, localize["InvalidModel"], StatusCodes.Status400BadRequest, true);
                }

                var parameter = await repository.GetFirstOrDefaultAsync(predicate: q => q.Id == parameterDto.Id);

                if (parameter != null)
                {
                    parameter.CdParam = parameterDto.CdParam;
                    parameter.CdPlant = parameterDto.CdPlant;
                    parameter.ChDescr = parameterDto.ChDescr;
                    parameter.ChParam = parameterDto.ChParam;
                    parameter.NoOrder = parameterDto.NoOrder;
                    parameter.SwActive = parameterDto.SwActive;
                    parameter.SwAllowDel = parameterDto.SwAllowDel;
                    parameter.SwAllowNew = parameterDto.SwAllowNew;
                    parameter.SwAllowUpd = parameterDto.SwAllowUpd;
                    parameter.SwParent = parameterDto.SwParent;

                    await repository.UpdateAsync(parameter).ConfigureAwait(false);

                    var updatedParameter = await repository.GetFirstOrDefaultAsync(predicate: p => p.Id == parameterDto.Id);
                    var dto = mapper.Map<Domain.Definition.Parameter, ParameterDto>(updatedParameter);

                    return serviceResponseHelper.SetSuccess(dto);
                }

                return serviceResponseHelper.SetError<ParameterDto>(null, localize["ModelIsNotFound"], StatusCodes.Status400BadRequest, true);
            }
            catch (Exception ex)
            {
                return serviceResponseHelper.SetError<ParameterDto>(null, ex.InnerException?.Message ?? ex.Message, StatusCodes.Status500InternalServerError, true);
            }
        }

And this is my unit test for this method

[Fact]
        public async Task ParameterService_Update_ReturnsUpdatedParameterDto()
        {
            // Arrange
            var fixture = new Fixture();
            var parameterDto = fixture.Create<ParameterDto>();
            var parameter = fixture.Build<Domain.Definition.Parameter>()
                .With(p => p.Id, parameterDto.Id)  // Set the same Id as the parameterDto for matching
                .Create();
            var updatedParameter = fixture.Create<Domain.Definition.Parameter>();
            var expectedResponse = new ServiceResponse<ParameterDto>(fixture.Create<ParameterDto>());

            _repository.GetFirstOrDefaultAsync(Arg.Any<Expression<Func<Domain.Definition.Parameter, bool>>>(), null, null, default)
                .Returns(parameter);
            _repository.UpdateAsync(Arg.Any<Domain.Definition.Parameter>())
                .Returns(Task.FromResult(updatedParameter));
            _repository.GetFirstOrDefaultAsync(Arg.Any<Expression<Func<Domain.Definition.Parameter, bool>>>(), null, null, default)
                .Returns(updatedParameter);
            _mapper.Map<Domain.Definition.Parameter, ParameterDto>(Arg.Any<Domain.Definition.Parameter>())
                .Returns(expectedResponse.Result);

            // Act
            var result = await _parameterService.Update(parameterDto);

            // Assert
            result.Should().BeEquivalentTo(expectedResponse);
            result.Result.Should().BeEquivalentTo(expectedResponse.Result);
            result.Should().NotBeNull();
            result.IsSuccessful.Should().BeTrue();
        }

Error is coming from in the main code

var parameter = await repository.GetFirstOrDefaultAsync(predicate: q => q.Id == parameterDto.Id);

As i understand GetFirstOrDefaultAsync() methods need all parameters as expected instead of null parameters. I filled parameters and this worked for me.