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

This can happen when a Style is pointing to a StaticResource that does NOT exist.

This xaml was failing:

<Grid.Resources>
    <Style TargetType="{x:Type TextBox}">
        <Setter Property="Height" Value="{StaticResource StandardControlHeight}"/>
        <Setter Property="VerticalContentAlignment" Value="Center"/>
    </Style>
</Grid.Resources>

The error was:

System.InvalidOperationException: ''{DependencyProperty.UnsetValue}' is not a valid value for property 'Height'.'

When I added the missing StaticResource, the problem went away.

my guess is that the control that gets the focus when you close the mentioned window has a custom style set by you that does not include any FocusVisualStyle.

so to help you further, you should explain a bit more: what happens (or should happen) when you close this window?

what control type is supposed to get the focus?

Yet another way to cause mentioned exception is when you declare StaticResource after using it, for example in style declaration.

WRONG

<Style TargetType="Label">
    <Setter Property="Foreground" Value="{StaticResource BlueAccent}"/>
</Style>

<SolidColorBrush x:Key="BlueAccent" Color="#22afed"/>

CORRECT

<SolidColorBrush x:Key="BlueAccent" Color="#22afed"/>

<Style TargetType="Label">
    <Setter Property="Foreground" Value="{StaticResource BlueAccent}"/>
</Style>