NullReferenceException but checking for Null?

private async void SaveCourseButton_Clicked(object sender, EventArgs e)
{
    Course course = new Course(id, CourseName.Text, StartDate.Date, EndDate.Date, StatusPicker.SelectedIndex, InstructorName.Text, InstructorPhone.Text, InstructorEmail.Text, CourseNotes.Text, 0, 0);
    var result = DataObjects.ValidateCourseInfo(activeTerm, course);
    if (result.Length == 0)
    {
        SQLFunctions.AddNewCourse(course);
        await Navigation.PopAsync();
    }
    else
    {
        await DisplayAlert("ERROR", result, "OK");
    }
}

public static string ValidateCourseInfo(Term term, Course course)
{
    if (course.CourseName.Length == 0 || course.CourseName is null)
    {
        return "Course must have a name.";
    }
    else if (course.EndDate.Date < course.StartDate.Date)
    {
        return "End date must be equal or greater than start date.";
    }
    else if (course.StartDate.Date < term.StartDate || course.StartDate.Date > term.EndDate || course.EndDate.Date < term.StartDate || course.EndDate.Date > term.EndDate)
    {
        return "Course start and end dates must fall within term start and end dates.";
    }
    else if (course.Status == -1)
    {
        return "Must pick a course status.";
    }
    else if (course.InstructorName.Length == 0 || course.InstructorName is null)
    {
        return "Instructor must have a name.";
    }
...
    else
    {
        return string.Empty;
    }
}