Compatibility issue with XUnit's `IClassFixture` about nsubstitute HOT 2 CLOSED

public class Tests
{
  private readonly Fixture _fixture;

  public Tests()
  {
    _fixture = new Fixture();
  }

  //1.
  [Fact]
  public void ExistsIsCalledWhenValidationIsSuccessful()
  {
    //first we make sure that Validate() passes so that Exists() gets called
    _fixture.SharedSubstitute
      .Validate(Arg.Any<int>())
      .Returns(true);

    //act:
    _fixture.App.CheckIfNumberExists(1);

    //then we test that the substitute received a call to Exists() 
    _fixture.SharedSubstitute.Received().Exists(Arg.Any<int>());
  }

  //2.
  [Fact]
  public void ExistsIsNotCalledWhenValidationFails()
  {
    //now we pretend that Validate() fails, i.e. returns false.
    _fixture.SharedSubstitute
      .Validate(Arg.Any<int>())
      .Returns(false);

    //act:
    _fixture.App.CheckIfNumberExists(1);

    //we shouldn't expect a call to Exists()
    _fixture.SharedSubstitute.DidNotReceive().Exists(Arg.Any<int>());
  }
}