Excluding null reference when selecting comboboxes

Let me explain my case. I have a custom wpf control in which I am dynamically generating a combobox. For this purpose I am using the Class Control Service, from which I create comboboxes with their comboboxitems and also manage the selection change event for each of them. Everthing works great until I try to raise a second event on the first combo box. The problem comes when I try to clear the elements of the second group. I disabled debubugg and there is no problem with the combobox link I already checked (childCmB! = Null) but after I tried to clear the elements of that dropdown. an exception is thrown. I am trying to find many cases in the forums, but with any success, so I would appreciate any suggestion to resolve this issue.

thank

ControlService.class:

 static class ControlService
{
    private static ObservableDictionary<String, System.Windows.UIElement> controls = new ObservableDictionary<String, System.Windows.UIElement>();
    private static Grid expanderContent;

    public static Expander createExpander(String header, String name, String code)
    {
        header = manageControlName(header);
        ExpanderModelView expanderModelView = new ExpanderModelView(name);

        //Recieve template grid
        expanderContent = expanderModelView.getExpanderContent();
        //Add in static collection
        Expander expander = new Expander() { Header = header };
        //Code of provider service
        expander.Uid = code;
        expander.Name = name;

        //Set up handler
        expanderContent.AddHandler(System.Windows.Controls.ComboBox.SelectionChangedEvent, new SelectionChangedEventHandler(cmb_SelectionChanged));
        //Insert data
        expander.Content = expanderContent;

        controls.Add(header, expander);
        return expander;
    }

    public static void cleanUIElementCollection()
    {
        controls.Clear();
    }




    private static string manageControlName(string name)
    {
        int counter = 1;
        if (controls.Count != 0)
        {
            foreach (String value in controls.Keys)
            {
                if (value.Equals(name))
                {
                    if (name.Any(char.IsDigit))
                        name = name.Substring(0, name.IndexOf("("));

                    name = String.Format(name + "({0})", counter);
                    counter++;
                }
            }
        }
        return name;
    }

    private static void cmb_SelectionChanged(object sender, SelectionChangedEventArgs e)

    {
        Grid currentContent = sender as Grid;
        String selectedItemId = ((e.Source as System.Windows.Controls.ComboBox).SelectedItem as ComboBoxItem).Uid;
        IEnumerable<Parameter> childCmBs = DataService.getServiceParametersByParentId((e.Source as System.Windows.Controls.ComboBox).Uid);

        if (childCmBs.Any())
        {
            foreach (Parameter param in childCmBs)
            {
                try
                {

                System.Windows.Controls.ComboBox childCmB = currentContent.FindName(param.Key) as System.Windows.Controls.ComboBox;
                if (childCmB != null)
                {
                    childCmB.Items.Clear();
                    addComboBoxItems(DataService.getChildrenParameterValuesByParentId(selectedItemId), childCmB);
                }
                }
                catch (Exception ex)
                {
                    System.Windows.MessageBox.Show(String.Format("Happen some problem during creating comboboxes: {0}", ex.Message));
                }
            }
        }
    }

    public static void createComboBoxes(IEnumerable<Parameter> parameters, System.Windows.Controls.Panel panel)
    {
        foreach (Parameter parameter in parameters)
        {
            TextBlock cmbLabel = new TextBlock() { Text = parameter.Caption };
            System.Windows.Controls.ComboBox cmb = new System.Windows.Controls.ComboBox() { Name = parameter.Key, Uid = parameter.ID };
            panel.Children.Add(cmbLabel);
            panel.Children.Add(cmb);

            addComboBoxItems(parameter.ParameterValue, cmb);
        }
    }

    public static void addComboBoxItems(IEnumerable<ParameterValue> parameterValues, System.Windows.Controls.ComboBox cmb)
    { 
        foreach (ParameterValue value in parameterValues)
        {
            ComboBoxItem item = new ComboBoxItem()
            {
                Content = value.Caption,
                Uid = value.ID,
                Name = value.Key
            };
            cmb.Items.Add(item);
        }
    }

}