In code:
"<Window x:Class="FifteenGame.Wpf.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:FifteenGame.Wpf" xmlns:viewmodels="clr-namespace:FifteenGame.Wpf.ViewModels"
        mc:Ignorable="d"
        Title="Игра 15" Height="550" Width="800" Loaded="Window_Loaded">
    <Window.DataContext>
        <viewmodels:MainWindowViewModel/>
    </Window.DataContext>
    
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition />
        </Grid.RowDefinitions>

        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>

        <Grid Grid.Row="0" Grid.Column="1">
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>

            <StackPanel Grid.Row="0" Orientation="Horizontal">
                <TextBlock Text="Игрок:" FontSize="18" Margin="5" />
                <TextBlock Text="{Binding UserName}" FontSize="18" Margin="5" />
            </StackPanel>

            <StackPanel Grid.Row="1" Orientation="Horizontal">
                <TextBlock Text="Число ходов:" FontSize="18" Margin="5" />
                <TextBlock Text="{Binding MoveCountText}" FontSize="18" Margin="5" />
            </StackPanel>
        </Grid>

        <ItemsControl Grid.Column="1" Grid.Row="1" ItemsSource="{Binding Cells}">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="100" />
                            <RowDefinition Height="100" />
                            <RowDefinition Height="100" />
                            <RowDefinition Height="100" />
                        </Grid.RowDefinitions>

                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="100" />
                            <ColumnDefinition Width="100" />
                            <ColumnDefinition Width="100" />
                            <ColumnDefinition Width="100" />
                        </Grid.ColumnDefinitions>
                    </Grid>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>

            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Grid MouseDown="Grid_MouseDown" Tag="{Binding Direction}">
                        <Rectangle Margin="5" Stroke="Brown" StrokeThickness="3" RadiusX="10" RadiusY="10" Fill="LightBlue" />
                        <TextBlock Text="{Binding Text}" FontSize="36" HorizontalAlignment="Center" VerticalAlignment="Center" />
                    </Grid>
                </DataTemplate>
            </ItemsControl.ItemTemplate>

            <ItemsControl.ItemContainerStyle>
                <Style TargetType="ContentPresenter">
                    <Setter Property="Grid.Row" Value="{Binding Row}" />
                    <Setter Property="Grid.Column" Value="{Binding Column}" />
                </Style>
            </ItemsControl.ItemContainerStyle>
        </ItemsControl>
    </Grid>
</Window>
"

There is a problem in this lines:
"<Window.DataContext>
    <viewmodels:MainWindowViewModel/>
</Window.DataContext>"
IDE says
Ошибка	XDG0003	Ссылка на объект не указывает на экземпляр объекта.
Tell how to fix it