How to calculate TVirtualStringTree node data field

I have VirtualStringTree (VST) with nodes that have the next data structure :

Type
  PMyNodeData = ^TMyNodeData;
  TMyNodeData = record
    Cost:UnicodeString; 
  end;

The Child nodes which they don't have any children has Cost value but any parent nodes has cost value equaled to 0 and am trying to write procedure to make any parent node cost value equaled to sum of all its children cost .

I have tried the next :

procedure CalculateSum(Node:PVirtualNode);
var
  Data: PMyNodeData;
  ChildNode: PVirtualNode;
  ChildData: PMyNodeData;
  CostSum: Extended;
begin
  Data := VST.GetNodeData(Node);
  CostSum := 0.0;
  ChildNode := VST.GetFirstChild(Node);
  while Assigned(ChildNode) do
  begin
    ChildData := VST.GetNodeData(ChildNode);
    CostSum := CostSum + StrToFloat(ChildData.Cost);
    CalculateSum(ChildNode);
    ChildNode := VST.GetNextSibling(ChildNode);
  end;
  if  VST.HasChildren[Node] and (StrToFloat(Data.Cost) = 0) then
    Data.Cost := FloatToStr(CostSum);

end; 

The use :

CalculateSum(vst.RootNode);

But I get Access violation & the sum is not correct .. any suggestions.

I have solved the issue as the next :

    procedure CalculateSum(Node: PVirtualNode);
var
  Data: PMyNodeData;
  ChildNode: PVirtualNode;
  ChildData: PMyNodeData;
  CostSum: Extended;
begin
  Data := VST.GetNodeData(Node);
  if data<>nil then
  CostSum := Data.Cost else Costsum:=0.0;
  ChildNode := VST.GetFirstChild(Node);
  while Assigned(ChildNode) do
  begin
    ChildData := VST.GetNodeData(ChildNode);
    if ChildData <> nil then
    begin
      CalculateSum(ChildNode);
      CostSum := CostSum + ChildData.Cost;
    end;
    ChildNode := VST.GetNextSibling(ChildNode);
  end;
  if (VST.HasChildren[Node]) and (Data<>nil) then
    Data.Cost := CostSum;

end;