Hi,
the NullReferenceEception happens, if there are cells with empty text.
I changed line 38 of Form1.cs of your example to
dr[j] = (i == 3) ? "" : string.Format("row{0} col{1}", i, j);
Now when pressing the filter symbol does nothing. Clicking in the text area of the filter cell raises the exeption.
Greetings,
Thomas Beck