'Background' property does not point to a DependencyObject in path '(0).(1)'.'

[chrisdentremont]

When I use ShowDialog() to show my WPF window, I am getting this error. I imported what I needed into the XAML, and the preview window is displaying the window correctly, but the exception is thrown when the code tries to open the window.

This is the full XAML of the window:

<Window x:Class="EMD_Addin_Manager.Windows.AddinSelectWindow"
        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:EMD_Addin_Manager.Windows"
        xmlns:ui="http://schemas.modernwpf.com/2019"
        ui:WindowHelper.UseModernWindowStyle="True"
        mc:Ignorable="d" WindowStartupLocation="CenterScreen"
        Title="Select Addins To Export" Height="450" Width="400" ResizeMode="NoResize">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ui:ThemeResources RequestedTheme="Light" AccentColor="Green"/>
                <ui:XamlControlsResources />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="50" />
        </Grid.RowDefinitions>

        <ListBox Grid.Row="0" Name="Addins_ListBox" SelectionMode="Extended" Margin="4" />

        <Grid Grid.Row="1">
            <StackPanel Orientation="Horizontal" HorizontalAlignment="Left" Margin="4">
                <Button Name="Select_All_Button" Height="25" Width="75" Content="Select All" Margin="4" Click="Select_All_Button_Click" />
                <Button Name="Select_None_Button" Height="25" Width="75" Content="Select None" Margin="4" Click="Select_None_Button_Click" />
            </StackPanel>

            <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Margin="4">
                <Button Name="OK_Button" Height="25" Width="75" Content="OK" Margin="4" Click="OK_Button_Click" />
                <Button Name="Cancel_Button" Height="25" Width="75" Content="Cancel" Margin="4" Click="Cancel_Button_Click" />
            </StackPanel>
        </Grid>
    </Grid>
</Window>