Illustrator SDK :: Accessing Document From A Panel

I created a panel by following the EmptyPanel sample. Basically, my code looks like:
 
AIErr Plugin::AddWidgets()
{
  ...
  AIPanelPlatformWindow hDlg = NULL;
  sAIPanel->GetPlatformWindow(fPanel, hDlg);
  HWND HwndSelectButton = CreateWindowEx(
    0, "BUTTON",   // Predefined class; Unicode assumed.

[Code]...

The problem is, when I call sAIDocument->GetDocumentFileSpecification, I get a NULL file path with an error code of kNoDocumentErr.