ShowDialog() or Show() leads to NullReferenceException

[patrickiel]

I need to instantiate a window from a class library, I don't have any App.xaml...
I'm having problems instantiating the Window, as soon as I get to the ShowDialg() methode the view and viewmodel variable is null, I get a NullReferenceException.
I inherit from UiWindow (in MyView.xaml.cs) is that correct?

var model= new Model();
var viewModel = new ViewModel(model);
var view = new MyWindow(viewModel);
view.ShowDialog();

MyView.xaml.cs

public partial class ViewModel: UiWindow
{
    public MyWindow(ViewModel viewModel)
    {
        InitializeComponent();
        DataContext = viewModel;
    }
}

MyView.xaml

<ui:UiWindow x:Class="Views.MyView"
             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:vm="clr-namespace:ViewModels"
             xmlns:local="clr-namespace:Views"
             xmlns:ui="http://schemas.lepo.co/wpfui/2022/xaml"
             mc:Ignorable="d"
             d:DataContext="{d:DesignInstance vm:ViewModel, IsDesignTimeCreatable=False}"
             Title="my app"
             SizeToContent="WidthAndHeight"
             d:Background="{DynamicResource ApplicationBackgroundBrush}"
             Foreground="{DynamicResource TextFillColorPrimaryBrush}"
             ExtendsContentIntoTitleBar="True"
             WindowBackdropType="Mica"
             WindowCornerPreference="Round"
             wpfhelpers:WindowCloser.EnableWindowClosing="True">

    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ui:ThemesDictionary Theme="Dark" />
                <ui:ControlsDictionary />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    <Grid>
      <!-- ... -->
    </Grid>
</ui:UiWindow>