我为 Virtual StringTree 控件的 OnGetHint 事件写了一个处理程序，我确定返回了提示文本，但是提示窗口是空白的，看下面的截图，可能是什么导致了这个问题？先感谢您。
控件的设置：
AnimationDuration := 0;
ClipboardFormats.Clear;
ClipboardFormats.Add('HTML Format');
ClipboardFormats.Add('Plain text');
ClipboardFormats.Add('Rich Text Format');
ClipboardFormats.Add('Rich Text Format Without Objects');
ClipboardFormats.Add('Unicode text');
ClipboardFormats.Add('Virtual Tree Data');
DefaultPasteMode := amInsertBefore;
DragOperations := [doCopy, doMove, doLink];
EditDelay := 200;
HintMode := hmHint;
Images := dmImages.lstProjectTree;
IncrementalSearch := isAll;
Indent := 16;
Margin := 0;
ParentFont := False;
PopupMenu := pmTree;
SelectionCurveRadius := 5;
OnCollapsed := treePrjCollapsed;
OnColumnClick := treePrjColumnClick;
OnDblClick := treePrjDblClick;
OnDragAllowed := treePrjDragAllowed;
OnDragOver := treePrjDragOver;
OnDragDrop := treePrjDragDrop;
OnExpanded := treePrjExpanded;
OnFocusChanged := treePrjFocusChanged;
OnGetCursor := treePrjGetCursor;
OnGetText := treePrjGetText;
OnPaintText := treePrjPaintText;
OnGetImageIndexEx := treePrjGetImageIndexEx;
OnGetHint := treePrjGetHint;
OnKeyDown := treePrjKeyDown;
OnMouseDown := treePrjMouseDown;
OnNewText := treePrjNewText;
OnNodeMoving := treePrjNodeMoving;
with Columns.Add do begin
Position := 0;
Width := 153;
end;
with Columns.Add do begin
Layout := blGlyphRight;
MinWidth := 20;
Position := 1;
Width := 20;
end;
with Columns.Add do begin
Layout := blGlyphRight;
MinWidth := 20;
Position := 2;
Width := 20;
end;
with Columns.Add do begin
Layout := blGlyphRight;
MinWidth := 16;
Position := 3;
Width := 20;
end;