How to use a Viewbox as a Ressource-Collection of common programming errors

I’ m trying to use a Viewbox as a Resource. See vbIconPcoGoToCurve in the XAML. If I’m using this Resource on one button only everything is fine. But when I use it on a Second button2 I get a Exception at Runtime. (In the Designer it looks ok, but in Runtime theres the exception).

Full code is here: