why are you using WindowsFormsHost to load WPF usercontrol?
Looks like you are trying to do as like example available in this
link[
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If you trying to load WPF UserControl no need to use WindowsFormHost.
<Window x:Class="WpfApplication1LL_Neu.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:lowLightGrab="clr-namespace:LowLightGrab;assembly=LowLightGrab"
Title="MainWindow" Height="350" Width="525">
<Grid>
<lowLightGrab:UserControl1/>
</Grid>
</Window>
the above code is enough If the lowlightgrab:usercontrol1 is a WPF UserControl