Created Unassigned: Comment.Delete() crashes workbook [9415]

Hi!

I'm troubled by a simple problem.
Sometimes, workbook including comments is crashed by deleting comment.
In order to ascertain reproducibility conditions, I prepared simple program.

```
class Program
{
static void Main(string[] args)
{
var book = new XLWorkbook();
book.AddWorksheet("comment");
book.Worksheet("comment").Cell(1, 1).Comment.AddText("test");
book.SaveAs("test.xlsx");

book = new XLWorkbook("test.xlsx");
book.Worksheet("comment").Cell(1, 1).Comment.Delete();
book.Save();
}
}
```

When I open this workbook in MS EXCEL, dialog appears for repairs workbook.
Does delete comments not available?