gridpanel: combobox in column editor not showing after edit

[lapix]

Hello, I have an issue with a combobox inside a column;
I have a gridpanel with a button to add rows client side; when I add a row there's a combobox with some values, correctly shown.
I select a value from the combo, but when I m done the combo is not shown, nor value or text of it, but the column has value since I can see the small modified red indicator (see image attached).
I have another column with textfield working properly in the secondo column.
I use a renderer script to show the combo text, but apparently the problem is not that one.
when I go back to a previously edited row, when I double click the cell the combo appears and show the correct previously selected value, so it looks like a display issue...

here's the code for gridpanel and combobox:

                 <ext:GridPanel 
                        runat="server" 
                        ID="grdVarieta" 
                        Title="varietà" 
                        StripeRows="true" 
                        AutoExpandColumn="clVarieta">
                    <Store>
                    <ext:Store runat="server" ID="storeVarieta" >
                    <Reader>
                    <ext:JsonReader IDProperty="IdPif08">
                        <Fields>
                        <ext:RecordField Name="varieta" ServerMapping="IdXst01Pif08.Value" />
                        <ext:RecordField Name="nrPiante" ServerMapping="Qta.Value"/>
                        </Fields>
                    </ext:JsonReader>
                    </Reader>
                    </ext:Store>
                    </Store>
                    <ColumnModel runat="server" >
                        <Columns>
                        <ext:Column Header="Varietà"  ColumnID="clVarieta" DataIndex="varieta" >
                         <Renderer Fn="Renderer" />
                        <Editor>
                        <ext:ComboBox 
                            runat="server" 
                            ID="cbVar" 
                            ForceSelection="true" 
                            TriggerAction="All"
                            DisplayField="DescrizioneXst01"
                            ValueField="IdXst01">
                            <Store>
                            <ext:Store runat="server" ID="storeCbVar" >
                            <Reader>
                                <ext:JsonReader>
                                <Fields>
                                <ext:RecordField Name="IdXst01" ServerMapping="IdXst01.Value" />
                                <ext:RecordField Name="DescrizioneXst01" ServerMapping="DescrizioneXst01.Value" />
                                </Fields>
                                </ext:JsonReader>
                            </Reader>
                            </ext:Store>
                            </Store>
                        </ext:ComboBox>
                        </Editor>
                        </ext:Column>
                        <ext:NumberColumn Header="Numero piante" DataIndex="nrPiante" Format="000">
                            <Editor>
                                <ext:TextField runat="server" ID="txtNrPiante" />
                            </Editor>
                        </ext:NumberColumn>
                        </Columns>
                    </ColumnModel>
                     <SelectionModel>
                        <ext:RowSelectionModel ID="RowSelectionModel1" runat="server" />
                    </SelectionModel>
                    <Buttons>
                    <ext:Button runat="server" ID="btnAddVarieta" Icon="Add" Text="Aggiungi varietà">
                    <Listeners>
                        <Click Handler="var rowIndex = #{grdVarieta}.addRecord(); #{grdVarieta}.getView().focusRow(rowIndex); #{grdVarieta}.startEditing(rowIndex, 0);" />
                    </Listeners>
                    </ext:Button>
                    </Buttons>
                    </ext:GridPanel>

this is the code for the renderer:

    <ext:ResourcePlaceHolder ID="ResourcePlaceHolder2" runat="server" />
        <script type="text/javascript">
           
        var Renderer = function (value) {
            var r = storeCbVar.getById(value);

            if (Ext.isEmpty(r)) {
                return "";
            }

            return r.data.DescrizioneXst01;
        };

    </script>

Can someone tell me why the combo is not shown after edit?
Many thanks in advance

[lapix]

Solved... forgot idProperty in the combo store reader.