This looks like a bug. I would recommend submitting the issue to the support group: http://es.infragistics.com/gethelp. If you need to work around this issue, the best way would probably be to remove the controls from your ControlContainerTools.
This UltraWinToolbar contains PopupMenuTool and ControlContainerTool. When I risize the form by dragging from right-down corner, when form size starts to get less than toolbar size, ToolBar gets invisible also the PopupMenuTool that it contains gets invisible but the ControlContainerTool still remains there. When I click on the ControlContainerTool I get above exception. Probably because ControlContainerTool's parent UltraWinToolbar is not visible.
Is there any way to solve this issue? I expect the toolbar's childs to become invisible when toolbar itself is not visible.
Thanks for help.
Its difficult to say why the control isn't being hidden. You should report the issue to the support group so it can be investigated further.