"The name does not exist in the namespace" in XAML designer

Question

I'm trying bind my Classes to HierarchicalDataTemplate. See code below.

XAML

<Window x:Name="window" x:Class="MyCompany.Press.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:helpers="clr-namespace:MyCompany.Press.Helpers"
        xmlns:press="clr-namespace:MyCompany.Press"
        mc:Ignorable="d"
        Title="Press analyzer" Height="500" Width="800.5" ContentRendered="WindowContentRenderedEventHendler" Closing="WindowClosingEventHandler"
        d:DataContext="{d:DesignData MainWindow}">
    <Grid Name="MainGrid">
            ...
            <TreeView Name="ArticlesTreeView" Grid.Column="0" AllowDrop="True" >
                <TreeView.Resources>
                    <HierarchicalDataTemplate DataType="{x:Type press:NewsPaperDocument}" ItemsSource="{Binding Children}">
                        <TextBlock Text="{Binding Name}" Tag="{Binding Object}" FontWeight="Bold" />
                    </HierarchicalDataTemplate>
                    <HierarchicalDataTemplate DataType="{x:Type press:NewsPaperPage}" ItemsSource="{Binding Children}">
                        <TextBlock Text="{Binding Name}" Tag="{Binding Object}" Foreground="#00a300" />
                    </HierarchicalDataTemplate>
                    <HierarchicalDataTemplate DataType="{x:Type press:NewsPaperTitle}" ItemsSource="{Binding Children}">
                        <TextBlock Text="{Binding Name}" Tag="{Binding Object}" Foreground="#da532c" />
                    </HierarchicalDataTemplate>
                    <DataTemplate DataType="{x:Type press:NewsPaperBlock}">
                        <TextBlock Text="{Binding Name}" Tag="{Binding Object}" Foreground="#2b5797" />
                    </DataTemplate>
                </TreeView.Resources>
                <TreeView.ItemContainerStyle>
                    <Style TargetType="{x:Type TreeViewItem}">
                        <!-- Style for the selected item -->
                        <Setter Property="BorderThickness" Value="1" />
                        <Style.Triggers>
                            <!-- Selected and has focus -->
                            <Trigger Property="IsSelected" Value="True">
                                <Setter Property="BorderBrush" Value="#7DA2CE" />
                            </Trigger>
                            <!-- Mouse over -->
                            <Trigger Property="helpers:TreeViewHelper.IsMouseDirectlyOverItem" Value="True">
                                <Setter Property="Background">
                                    <Setter.Value>
                                        <LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
                                            <GradientStop Color="#FFFAFBFD" Offset="0" />
                                            <GradientStop Color="#FFEBF3FD" Offset="1" />
                                        </LinearGradientBrush>
                                    </Setter.Value>
                                </Setter>
                                <Setter Property="BorderBrush" Value="#B8D6FB" />
                            </Trigger>
                            <!-- Selected but does not have the focus -->
                            ...
                        </Style.Triggers>
                        <Style.Resources>
                            <Style TargetType="Border">
                                <Setter Property="CornerRadius" Value="2" />
                            </Style>
                        </Style.Resources>
                    </Style>
                </TreeView.ItemContainerStyle>
            </TreeView>
        </Grid>
    </Grid>
</Window>

If I use x:Type, designer say

Design view is unavailable for x64 and ARM target platforms.

and error like

The name "NewsPaperDocument" does not exist in the namespace "clr-namespace:MyCompany.Press"

but code compile and run completely.

Same error I have on <Trigger Property="helpers:TreeViewHelper.IsMouseDirectlyOverItem" Value="True"> line. If I remove x:Type designer works fine, but HierarchicalDataTemplate doesn't work correctly

P.S. TreeViewHelper I get here http://blogs.msdn.com/b/mikehillberg/archive/2006/09/21/mytreeviewhelperismousedirectlyoveritem.aspx