NullReference Exception on non-null object

Question

When I pass an Exam object to this viewmodel constructor, both WriteLine calls behave as expected, indicating that exam (and Exam) is not null.

(I put the WriteLines in because I was trying to bind to {Binding Exam.Name} in my view and nothing )was coming in.

public ExamInfoViewModel(Exam exam, RelayCommand startAcquisitionHandler, RelayCommand startEditHandler, RelayCommand backHandler)
{
    base.DisplayName = "Exam Info";
    Exam = exam;
    System.Console.WriteLine(Exam);
    System.Console.WriteLine(Exam.Name);
    StartAcquisitionCommand = startAcquisitionHandler;
    StartEditingCommand = startEditHandler;
    BackCommand = backHandler;
}

However, I get an error, highlighting the second WriteLine call.

System.NullReferenceException: 'Object reference not set to an instance of an object.' Gui.ViewModels.ViewModelWithExam.Exam.get returned null.

The other arguments to the constructor work, by the way. If I take out the WriteLine calls I don't get any errors, and the commands work as expected (well I've only actually implemented the BackCommand in my view, but that one works).

The code that calls the constructor:

private RelayCommand OnSelectExam => new RelayCommand(exam =>
{
    Exam = (Exam)exam;
    CurrentViewModel = new ExamInfoViewModel(
        Exam,
        OnRequestAcquireImages,
        OnRequestEditExam,
        new RelayCommand(_ => GoHome())
    );
});

And here's the base class:

public class ViewModelWithExam : ViewModelBase
{
    private Exam _exam;

    public Exam Exam
    {
        get => _exam;
        set
        {
            _exam = value;
            OnPropertyChanged();
        }
    }
}

UPDATE:

I put some console writers after the first line in OnSelectExam, and it turns out it runs twice, once with exam non-null, and then with it null. This appears to be what the actual problem is. I imagine the code from ExamsIndexViewModel, which calls OnSelectExam, will help:

internal class ExamsIndexViewModel : NavigableViewModel
{
    public ExamsIndexViewModel(RelayCommand newExamHandler, RelayCommand selectExamHander)
    {
        base.DisplayName = "Select Exam";

        NewExamCommand = newExamHandler;
        SelectExamCommand = selectExamHander;

        ExamsList = GetExams();
    }
    public RelayCommand NewExamCommand
    {
        get;
    }

    public RelayCommand SelectExamCommand
    {
        get;
    }

And from the class that instantiates the ExamsIndexViewModel:

private void GoHome()
{
    CurrentViewModel = new ExamsIndexViewModel(OnNewExam, OnSelectExam);
}

public ExamsTabViewModel() : base()
{
    base.DisplayName = "Exams";
    GoHome();
}