WPF - System.Windows.Markup.XamlParseException when Referencing a resource from App.xaml

I have defined a resource in App.xaml and assign it a key. I get System.Windows.Markup.XamlParseException exception when I reference the resource from a AddUser.xaml window in the app. The error occurs at line InitializeComponent(); (of AddUser.xamlcs file) that is called when I instantiate the AddUser object inside btnNewUser_Click(..) of Mainwindow.xaml.cs.

According to this response on a similar issue, the easiest way is to place the resource into App.xaml (that is what I'm actually doing). Question: What I may be missing here and how the issue can be resolved?

Remarks: As done here as well, you may notice that since I'm using Commandds, I'm using the namespace WpfApp1.Commands in App.xaml and App.xam.cs files.

Error:

System.Windows.Markup.XamlParseException
  HResult=0x80131501
  Message='Provide value on 'System.Windows.StaticResourceExtension' threw an exception.
.....
Inner Exception 1:
Exception: Cannot find resource named 'Auser'. Resource names are case sensitive.

App.Xaml:

<Application x:Class="WpfApp1.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp1.Commands"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <local:AddUser x:Key="Auser"/>
    </Application.Resources>
</Application>

MainWindow.xaml:

<Window x:Class="WpfApp1.MainWindow"
        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:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="Auto"/>
        </Grid.ColumnDefinitions>
        <Button x:Name="btnNewUser" Content="Create User" Click="btnNewUser_Click"/>
    </Grid>
</Window>

MainWindow.xaml.cs:

using System.Windows;
using WpfApp1.Commands;

namespace WpfApp1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void btnNewUser_Click(object sender, RoutedEventArgs e)
        {

            AddUser addUser = new AddUser();
            if(addUser.ShowDialog()==true)
               MyAddUser(addUser.UserName, addUser.Department);
        }

        private void MyAddUser(string sUserName, string sDepartment)
        {
          //will add code here
        }
    }
}

AddUser.xaml:

<Window x:Class="WpfApp1.Commands.AddUser"
        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:WpfApp1"
        mc:Ignorable="d"
        Title="AddUser" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <Label Content="User Name:"/>
        <TextBlock x:Name="txtUserName" Grid.Column="1">
            <TextBlock.Text>
                <Binding Path="UserName" Source="{StaticResource Auser}"/>
            </TextBlock.Text>
        </TextBlock>
        <Label Content="Department:" Grid.Row="1"/>
        <TextBox x:Name="txtDepartment" Grid.Row="1" Grid.Column="1">
            <TextBox.Text>
                <Binding Path="Department" Source="{StaticResource Auser}"/>
            </TextBox.Text>
        </TextBox>
        <Button x:Name="btnAdd" Content="Add User" Grid.Row="2" Grid.Column="1">
            <Button.CommandBindings>
                <CommandBinding Command="New" CanExecute="CommandNew_CanExecute" Executed="CommandNew_Executed"/>
            </Button.CommandBindings>
        </Button>
        
    </Grid>
</Window>

AddUser.xaml.cs:

NOTE: Error occurs at the line InitializeComponent(); when AddUser object is instantiated inside btnNewUser_Click(..) of Mainwindow.xaml.cs

using System.Windows;
using System.Windows.Input;

namespace WpfApp1.Commands
{
    public partial class AddUser : Window
    {
        public AddUser()
        {
            InitializeComponent();
        }

        public string UserName
        {
            get { return txtUserName.Text; }
        }

        public string Department
        {
            get { return txtDepartment.Text; }
        }

        private void CommandNew_CanExecute(object sender, CanExecuteRoutedEventArgs e)
        {

        }

        private void CommandNew_Executed(object sender, ExecutedRoutedEventArgs e)
        {

        }
    }
}