"InvalidOperationException: An animation is trying to modify an object named 'foo', but no such object can be found in the UserControl."
When you create screen with a tabcontrol on it and add some tabitems, then define some states to control visibility of UI controls inside a tabitem, you will get this error in Expression when you select a different tab.
You can sorta work around it by viewing the XAML and adding SelectedIndex="0" to the TabControl attributes and saving the XAML file. Then you can select the design surface and it will display. You can navigate the tabs until you try to go from the tab with animation UI elements to a different tab. Set SelectedIndex="0" and save to get back to the artboard.
The player does not seem to mind. There must be a way to define states and use the tab control cleanly