public void AllowSettingAssigneeToNull()
        {
            var issue = new Issue(new Project(new ApplicationUser(), "project"), "name", "description");

            issue.Assignee.Should().Be(null);
            issue.AssignUser(new ApplicationUser());
            issue.Assignee.Should().NotBe(null);
            issue.AssignUser(null);
            issue.Assignee.Should().Be(null);
        }