Creating HTML table from List throws null exception (Printing to console works)

Here is the method for generating HTML table:

   public static string CreateHTMLTable<T>(IEnumerable<T> list, params Func<T, (object, string)>[] fxns) // Used for generating HTML tables for Outlook
    {
        var sb = new StringBuilder();
        sb.Append("<table>\n");
        sb.Append("<thead>\n");
        sb.Append("<tr>\n");

        foreach (var fxn in fxns)
            sb.Append("<th>").Append(fxn(default).Item2).AppendLine("</th>");

        sb.Append("</tr>\n");
        sb.Append("</thead>\n");

        foreach (var item in list)
        {
            sb.Append("<tr>\n");
            foreach (var fxn in fxns)
            {
                sb.Append("<td>");
                sb.Append(fxn(item).Item1);
                sb.Append("</td>");
            }
            sb.Append("\n</tr>\n");
        }
        sb.Append("</table>");

        return sb.ToString();
    } 

I have Linq:

        var ComparedProjectsList = MainProcess.Projects.AsEnumerable()
            .GroupBy(x => x.Field<string>("PROJECTNUMBER").Trim())
            .Select(x => new
            {
                PROJECTNUMBER = x.Key,
                PROJECTNAME = x.Select(y => y.Field<string>("PROJECTNAME").Trim()),
                user = x.Select(y => y.Field<string>("PROJECTMANAGER").Trim()).Distinct().ToList()
            })
            .Where(x => x.user.Count > 1)
            .Select(x => new
            {
                PROJECTNUMBER = x.PROJECTNUMBER,
                PROJECTNAME = x.PROJECTNAME.First().ToString(),
                ProjectManager = x.user[0],
                ProjectManager2 = x.user[1]
            })
            .ToList();

Just for checking that there are actually values:

        foreach (var item in ComparedProjectsList)
        {
            Console.WriteLine("{0} - {1} {2} / {3}", item.PROJECTNUMBER, item.PROJECTNAME, item.ProjectManager, item.ProjectManager2);
        }

I get in console:

123456 - PROJECT 1 JOHN SMITH / DONALD RED
235454 - PROJECT 2 LIINOS TORVALDS / FUJI BANKS
456546 - PROJECT 3 STEVEN SEAGAL / TOM HAWK

I am using this for generating HTML table:

        var ComparedHTMLTableProject = CreateHTMLTable,
            x => (x.PROJECTNUMBER, "Project number"),
            x => (x.PROJECTNAME, "Sub-project number"),
            x => (x.ProjectManager, "Project manager"),
            x => (x.ProjectManager2, "Project manager 2")
            ); 

I am getting an exception:

System.NullReferenceException HResult=0x80004003 Message=Object reference not set to an instance of an object.
Source=DeliveryProjectAnalyzer StackTrace: at DeliveryProjectAnalyzer.Program.<>c.b__9_7(<>f__AnonymousType5`4 x) in x was null.

Why so? How x is null if it is printed to the console? I also don't understand at what point I should check for null as printing to Console works without any problems? In addition to that I have used CreateHTMLTable method before as well in similar cases for modifying List to HTML table without any issues.

For anyone who might be wondering the same, here is the solution:

    var ComparedHTMLTableProject = CreateHTMLTable,
        x => (x?.PROJECTNUMBER, "Project number"),
        x => (x?.PROJECTNAME, "Sub-project number"),
        x => (x?.ProjectManager, "Project manager"),
        x => (x?.ProjectManager2, "Project manager 2")
        );