Delphi用GetEntity方法取图元时发生错误，请问高手怎么弄呢？

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Delphi用GetEntity方法取图元时发生错误，请问高手怎么弄呢？
var
AcadApp: OleVariant;
AcadDoc: OleVariant;
AcadObj: AcadEntity;
picpnt:Olevariant;

begin
  try
     AcadApp:=GetActiveOleObject('AutoCAD.Application');
  except
     AcadApp:=CreateOleObject('AutoCad.Application');
  end;
AcadDoc := AcadApp.ActiveDocument;
acaddoc.utility.getentity (acadobj,picpnt,'选择1个图元：');
acadobj.color:=5;
end;

我这样编写为会报错，说发生意外,请问高手怎么解决呢？