Hi Team,
I am trying to use linked comboboxes.

Please have a look to the below code.

<ext:Store runat="server" ID="stddlSubCategory" AutoLoad="false" OnRefreshData="SubCatRefresh">
        <DirectEventConfig>
            <EventMask ShowMask="false" />
        </DirectEventConfig>
        <Reader>
            <ext:JsonReader IDProperty="SubCategoryId">
                <Fields>
                    <ext:RecordField Name="SubCategoryId" Mapping="SubCategoryId" />
                    <ext:RecordField Name="SubCategoryName" Mapping="SubCategoryName" />
                </Fields>
            </ext:JsonReader>
        </Reader>
        <Listeners>
            <Load Handler="#{SubCategory}.setValue(#{SubCategory}.store.getAt(0).get('SubCategoryId'));" />
        </Listeners>
    </ext:Store>
    <ext:Store ID="storeddlCategory" runat="server" AutoDataBind="true">
        <Reader>
            <ext:JsonReader IDProperty="CategoryId">
                <Fields>
                    <ext:RecordField Name="CategoryId" />
                    <ext:RecordField Name="CategoryName" />
                </Fields>
            </ext:JsonReader>
        </Reader>
    </ext:Store>
    

 <ext:SelectBox ID="ddlCategory" TriggerAction="All" runat="server" StoreID="storeddlCategory"
                FieldLabel="Category" DisplayField="CategoryName" ValueField="CategoryId" DataIndex="CategoryId"
                EmptyText="Select a category...">
                <Listeners>
                    <Select Handler="#{SubCategory}.clearValue(); #{stddlSubCategory}.reload();" />
                </Listeners>
            </ext:SelectBox>
            <ext:ComboBox ID="SubCategory" runat="server" FieldLabel="Sub-Category" StoreID="stddlSubCategory"
                TypeAhead="true" Mode="Local" ForceSelection="true" TriggerAction="All" DisplayField="SubCategoryName"
                ValueField="SubCategoryId" EmptyText="No Subcategory..." ValueNotFoundText="Loading...">
            </ext:ComboBox>

This is working fine when I am having subcategories(2nd dropdown) for the categories(1st dropdown).
But when there is no subcategory,
<Load Handler="#{SubCategory}.setValue(#{SubCategory}.st ore.getAt(0).get('SubCategoryId'));" />

I am getting exception,store.getAt(...) is null or not an object.

How can I check if no subcategories are loaded?or is there any other way?