EVariantTypeCastError Exception in DBGrids

[Vodnik]

EVariantTypeCastError Exception in DBGrids with message:
"Could not convert variant of type (Null) into type (Date)"

This occurs in TCustomDBGrid.InternalAutoSizeColumn, line 33:

Code: Pascal  [Select][+][-]

1. procedure TCustomDBGrid.InternalAutoSizeColumn(aCol: Integer; aCanvas: TCanvas; aDatalinkActive: Boolean);
2. var
3.   Field: TField;
4.   C: TGridColumn;
5.   ColWidth: Integer;
6.   ARow,w: Integer;
7.   s: string;
8.  
9. begin
10.   Field := GetFieldFromGridColumn(ACol);
11.   C := ColumnFromGridColumn(ACol);
12.  
13.   if (C<>nil) and (C.Title<>nil) then begin
14.     aCanvas.Font := C.Title.Font;
15.     ColWidth := aCanvas.TextWidth(trim(C.Title.Caption));
16.     aCanvas.Font := C.Font;
17.   end else begin
18.     if (Field<>nil) then begin
19.       aCanvas.Font := TitleFont;
20.       ColWidth := aCanvas.TextWidth(Field.FieldName);
21.     end
22.     else
23.       ColWidth := 0;
24.     aCanvas.Font := Font;
25.   end;
26.  
27.   if (Field<>nil) and aDatalinkActive then
28.     for ARow := FixedRows to RowCount-1 do begin
29.  
30.       FDatalink.ActiveRecord := ARow - FixedRows;
31.  
32.       if Field.dataType<>ftBlob then
33.          s := trim(Field.DisplayText)
34.       else
35.         s := '(blob)';
36.       w := aCanvas.TextWidth(s);
37.       if w>ColWidth then
38.         ColWidth := w;
39.  
40.     end;
41.  
42.   if ColWidth=0 then
43.     ColWidth := GetColumnWidth(ACol);
44.  
45.   ColWidths[ACol] := ColWidth + 15;
46. end;
47.  

Lazarus 1.8.0, FPC 3.0.4
I use TODBCconnection to Informix. I have tried my application on several different DB's, only one reports this error. This may be because of error in DB.
I'm not feeling brave enough to do proper correction in DBGrids source code.
Please help me to filter out this exception.

[Vodnik]

Well, non-trivial (for me) troubleshooting have brought me to the problem cause:
it was in my TSQLQuery.onGetText event handler. Corrected variant:
 

Code: Pascal  [Select][+][-]

1. procedure TDBForm.DBquery1callendGetText(Sender: TField; var aText: string; DisplayText: Boolean);
2. begin
3.   if Sender.IsNull then aText:='Null' else
4.     if (DisplayText) then
5.       aText := FormatDateTime('hh:nn:ss', Sender.Value / SecsPerDay)
6.     else aText := 'Unknown';
7. end;
8.  

Sendler.IsNull check is mandatory!!!

What is really strange, that this event handler is called even if query returns no data.