各位老师 救救我呵,我的delphi确程序 编译没有错,但运行出错

Question

各位老师delphi 编译没有错,但运行出错 
 运行时,先出现一个出错提示  " could convert variant of type (Null)  into type (string)"  做的是数据库记录的查询

Answer 1

  老师,加了这些代码运行就出错了.不知错在那里,烦您 帮助了.
implementation{$R *.dfm}procedure TForm6.Button1Click(Sender: TObject);
begin
    close;
end;procedure TForm6.FormCreate(Sender: TObject);
   var province,area:string[10];
      thisnode:TTreeNode;begin     Adoquery2.SQL.Clear;
     Adoquery2.SQL.Add('select DISTINCT  Ê¡·Ý from stuinfo');
     Adoquery2.Open;
     thisnode:=TreeView1.Items.GetFirstNode;
     while  not Adoquery2.Eof do
         begin
            province:=Adoquery2.Fields[0].AsString; //ÔÚADOquery2ÐòºÅΪ0
            thisnode:=TreeView1.Items.Add(thisnode,province);
            Adoquery3.SQL.Clear;
            ADoquery3.SQL.Add('select DISTINCT µØÇø from stuinfo WHERE Ê¡·Ý=:a');
            ADOquery3.Parameters.ParamByName('a').Value:=province;
            ADOquery3.Open;
            while not ADOquery3.Eof do
                begin
                    area:=ADOquery3.Fields[0].Value;
                    Treeview1.Items.AddChildFirst(thisnode,area);
                    Adoquery3.Next;
                end;
                ADoQuery2.Next;
          end;end;procedure TForm6.TreeView1Click(Sender: TObject);
     var province,area:string[10];begin
     if Treeview1.Selected.Level=0 then   //µ¥»÷µÄÊǵÚÒ»²ã½áµã£¬±íʾʡ·Ý
        begin                  //½«TreeView1Ñ¡ÖнáµãÖµ×÷ΪADOquery1µÄ²Î¿¼Öµ
            province:=Treeview1.Selected.Text;
            ADOquery1.SQL.Clear;
            if Province='È«²¿' then
                ADOquery1.SQL.Add('select  * from stuinfo')
                    else  begin
                        ADOquery1.SQL.Add('select * from stuinfo WHERE Ê¡·Ý=:a');
                        ADOquery1.Parameters.ParamByName('a').Value:=province;
                    end;
                ADoquery1.Open;
                end
                else  begin
                    province:=Treeview1.Selected.Parent.Text;
                    area:=Treeview1.Selected.Text;
                    Adoquery1.SQL.Clear;
                    ADOquery1.SQL.Add('select * from stuinfo Where (Ê¡·Ý=:a)and (µØÇø=:b)');
                    ADOquery1.Parameters.ParamByName('a').Value:=province;
                    ADoquery1.Parameters.ParamByName('b').Value:=area;
                    ADOquery1.Open;
                end;
end;end.

Answer 2

这是通过word 可以显示中文
procedure TForm6.FormCreate(Sender: TObject);
  var province,area:string[10];
      thisnode:TTreeNode;
begin
    Adoquery2.SQL.Clear;
     Adoquery2.SQL.Add('select DISTINCT  省份 from stuinfo');
     Adoquery2.Open;
     thisnode:=TreeView1.Items.GetFirstNode;
     while  not Adoquery2.Eof do
         begin
            province:=Adoquery2.Fields[0].AsString; //在ADOquery2序号为0
            thisnode:=TreeView1.Items.Add(thisnode,province);
            Adoquery3.SQL.Clear;
            ADoquery3.SQL.Add('select DISTINCT 地区 from stuinfo WHERE 省份=:a');
            ADOquery3.Parameters.ParamByName('a').Value:=province;
            ADOquery3.Open;
            while not ADOquery3.Eof do
                begin
                    area:=ADOquery3.Fields[0].Value;
                    Treeview1.Items.AddChildFirst(thisnode,area);
                    Adoquery3.Next;
                end;
   ADoQuery2.Next;
          end;
end;procedure TForm6.TreeView1Click(Sender: TObject);
    var province,area:string[10];
begin
    if Treeview1.Selected.Level=0 then   //单击的是第一层结点，表示省份
        begin                  //将TreeView1选中结点值作为ADOquery1的参考值
            province:=Treeview1.Selected.Text;
            ADOquery1.SQL.Clear;
            if Province='全部' then
                ADOquery1.SQL.Add('select  * from stuinfo')
                    else  begin
                        ADOquery1.SQL.Add('select * from stuinfo WHERE 省份=:a');
                        ADOquery1.Parameters.ParamByName('a').Value:=province;
                    end;
                ADoquery1.Open;
                end
                else  begin
                    province:=Treeview1.Selected.Parent.Text;
                    area:=Treeview1.Selected.Text;
                    Adoquery1.SQL.Clear;
                    ADOquery1.SQL.Add('select * from stuinfo Where (省份=:a)and (地区=:b)');
                    ADOquery1.Parameters.ParamByName('a').Value:=province;
                    ADoquery1.Parameters.ParamByName('b').Value:=area;
                    ADOquery1.Open;
                end;
        end;

Answer 3

设置断点了么?
area:=ADOquery3.Fields[0].Value;
这一句改成
area:=ADOquery3.Fields[0].AsString试试;