Fix ComboBoxPopup leak
Fix ComboBoxPopup leak

Make sure RemoveValueChanged is called on ComboBoxPopup.Child property

ButchersBoy/MaterialDesignInXamlToolkit,DingpingZhang/MaterialDesignInXamlToolkit,ButchersBoy/MaterialDesignInXamlToolkit,ButchersBoy/MaterialDesignInXamlToolkit,tipunch74/MaterialDesignInXamlToolkit,tipunch74/MaterialDesignInXamlToolkit,tipunch74/MaterialDesignInXamlToolkit