'{DependencyProperty.UnsetValue}' is not a valid value for property 'BorderBrush'.

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I've found that the reason of such error could be wrong resource dictionaries order in App.xaml. Always remember to put resource dictionaries declarations in App.xaml in order of use, so if you have two ResourceDictionaries A and B, and A uses some resources from B you must put it in App.xaml in following order: B, A.