Cannot set unknown member 'System.Windows.Controls.TextBox.IsReadOnly - wpf

Question

I am making a style for ComboBox and added this code to the ComboBox template
...
<TextBox x:Name="PART_EditableTextBox"
Style="{x:Null}"
Template="{StaticResource ComboBoxTextBox}"
HorizontalAlignment="Left"
VerticalAlignment="Bottom"
Margin="{TemplateBinding Padding}"
Focusable="True"
Background="Transparent"
Visibility="Hidden"
IsReadOnly="{TemplateBinding IsReadOnly}" />
...
The problem is in IsReadOnly="{TemplateBinding IsReadOnly}" property. Visual Studio tells that this is an unknown member and doesn't render a ComboBox in the designer. But if I launch the project, it will work properly. By the way, a ComboBox has property IsReadOnly. How to fix this glitch?
Upd: ComboBox template
<Style TargetType="{x:Type ComboBox}">
<Setter Property="SnapsToDevicePixels" Value="true" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
<Setter Property="ScrollViewer.CanContentScroll" Value="true" />
<Setter Property="MinWidth" Value="120" />
<Setter Property="MinHeight" Value="20" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ComboBox}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="20"/>
</Grid.ColumnDefinitions>
<ToggleButton x:Name="ToggleButton" Grid.ColumnSpan="2"
Template="{StaticResource ComboBoxToggleButtonTemplate}"
Focusable="False" ClickMode="Press"
IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
/>
<ContentPresenter x:Name="ContentSite" Grid.Column="0"
IsHitTestVisible="False"
Content="{TemplateBinding SelectionBoxItem}"
ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
Margin="{TemplateBinding Padding}"
VerticalAlignment="Stretch"
HorizontalAlignment="Left" />
<TextBox x:Name="PART_EditableTextBox"
Style="{x:Null}"
Template="{StaticResource ComboBoxTextBox}"
HorizontalAlignment="Left"
VerticalAlignment="Bottom"
Margin="{TemplateBinding Padding}"
Focusable="True"
Background="Transparent"
Visibility="Hidden"
IsReadOnly="{TemplateBinding IsReadOnly}" />
<!--IsReadOnly="{TemplateBinding IsReadOnly}"-->
<Border x:Name="Overlay" Grid.ColumnSpan="2" CornerRadius="5" Margin="-2" BorderThickness="3" BorderBrush="{StaticResource InputBorderFocused}" Visibility="Hidden"/>
<Popup x:Name="Popup"
Placement="Bottom"
IsOpen="{TemplateBinding IsDropDownOpen}"
AllowsTransparency="True"
Focusable="False"
PopupAnimation="Slide">
<Grid x:Name="DropDown" SnapsToDevicePixels="True" MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}">
<Border x:Name="DropDownBorder" BorderThickness="1"
BorderBrush="{DynamicResource InputBorder}"
Background="{DynamicResource InputBackground}"/>
<ScrollViewer Margin="0" SnapsToDevicePixels="True">
<StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="HasItems" Value="false">
<Setter TargetName="DropDownBorder" Property="MinHeight" Value="95" />
</Trigger>
<Trigger Property="IsGrouping" Value="true">
<Setter Property="ScrollViewer.CanContentScroll" Value="false" />
</Trigger>
<Trigger SourceName="Popup" Property="AllowsTransparency" Value="true">
<Setter TargetName="DropDownBorder" Property="Margin" Value="0,2,0,0" />
</Trigger>
<Trigger Property="IsFocused" Value="True">
<Setter Property="Visibility" Value="Visible" TargetName="Overlay"/>
</Trigger>
<Trigger Property="IsDropDownOpen" Value="True">
<Setter Property="Visibility" Value="Visible" TargetName="Overlay"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Answer 1

Late reply, but today I faced the same issue. What it fixed for me was just to set the PART_EditableTextBox.IsReadOnly through the Triggers of the ComboBox.Template. The TemplateBinding on the TextBox can be removed.
<ControlTemplate.Triggers>
<Trigger Property="IsReadOnly" Value="True">
<Setter TargetName="PART_EditableTextBox" Property="IsReadOnly" Value="True"></Setter>
</Trigger>
<Trigger Property="IsReadOnly" Value="False">
<Setter TargetName="PART_EditableTextBox" Property="IsReadOnly" Value="False"></Setter>
</Trigger>
<!--...-->
</ControlTemplate.Triggers>