高手看一下，access violation错误{form1.DBGrideh1.ColumnCount - 1}

Question

procedure Tform2.formcreate;
var
  I: Integer;
  Node: TdxTreeListNode;
begin
    TLfield.ClearNodes;
  for I := 0 to form1.DBGrideh1.ColumnCount - 1 do//运行到这一行                                             //access violation
  begin                                               // 错误， 
                                                      //提示form1=nil
    ......                                         
    Node := TLfield.Add;Node.Data := form1.DBGrideh1.Columns[I];
  end;
  end;