InvalidOperationException when closing a RadWindow opened on a separated UI thread. The exception message is: "Cannot use a DependencyObject that belongs to a different thread than its parent Freezable."
To work this around, you can merge the Telerik.Windows.Controls.Navigation.xaml dictionary in the Resources of all RadWindow instances open on another thread and then set their Style property to the RadWindowStyle from the merged .xaml dictionary.
<telerik:RadWindow x:Class="WpfApp84.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApp84" xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
mc:Ignorable="d" Height="450" Width="800" Style="{DynamicResource RadWindowStyle}">
<telerik:RadWindow.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/Telerik.Windows.Controls.Navigation;component/Themes/GenericWindows11.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</telerik:RadWindow.Resources>
</telerik:RadWindow>