Access violation at address ### in module 'vcl60.bpl'. Read of address ###

Hello experts,

I get the error "Access violation at address ### in module 'vcl60.bpl'. Read of address ###"
when running my application written in Borland c++ builder 6. (win2k).

The error occurs in the following function, just before returning the value, and only in the case that the return value points to NULL.

//-----------------------------------------------------------------------------------------
TTreeNode* TfrmNB::findNode(TTreeView* boom, int lvl, String srt, String tp, String nm){
  TTreeNode* n = boom->Items->GetFirstNode();
  switch (lvl){
    case 0:   // soort
      while (n && !(n->Text == srt)){
        n = n->getNextSibling();
      }
      break;
    case 1:   // type
      n = findNode(boom, 0, srt)->getFirstChild();
      while (n && !(n->Text == tp)){
        n = n->getNextSibling();
      }
      break;
    case 2:   // beheer
      n = findNode(boom, 1, srt, tp)->getFirstChild();
      while (n && !(n->Text == nm)){
        n = n->getNextSibling();
      }
      break;
  }
ShowMessage("returning : " + (n?n->Text:String("NULL")));
  return n;
}
//---------------------------------------------------------------------------

The message is displayed properly, and just after the functioncall, I had a showmessage ass wel, but that one is never shown. In stead, I got this annoying error.

The error only occurs when "the node was not found" (or NULL is returned).

Someone sugested to remove all desktops in the borland desktop manager, but I have no desktops at all...