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.