Привязка RichTextBox к свойству

<UserControl x:Class="SomeApp.Views.EditorView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:SomeApp.Views" 
             xmlns:res="clr-namespace:SomeApp.Resources"
             xmlns:vm="clr-namespace:SomeApp.ViewModels"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
    <!--<UserControl.CommandBindings>
        <CommandBinding Command="ApplicationCommands.Delete" Executed="Delete_Executed" />
        <CommandBinding Command="ApplicationCommands.Save" Executed="Save_Executed" />
    </UserControl.CommandBindings>-->
 
    <UserControl.DataContext>
        <vm:EditorVM/>
    </UserControl.DataContext>
 
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="0.07*"/>
            <RowDefinition Height="0.06*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <DockPanel Grid.Row="0" MaxHeight="40">
            <ToolBar DockPanel.Dock="Top">
                <ToggleButton Command="EditingCommands.ToggleBold" Name="btnBold" Content="Жирный" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
                <ToggleButton Command="EditingCommands.ToggleItalic" Name="btnItalic" Content="Курсив" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
                <ToggleButton Command="EditingCommands.ToggleUnderline" Name="btnUnderline" Content="Подчёркнутый" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
                <Separator />
                <ComboBox Name="cmbFontFamily" SelectionChanged="cmbFontFamily_SelectionChanged" Width="150" VerticalAlignment="Stretch"/>
                <ComboBox Name="cmbFontSize" IsEditable="True" TextBoxBase.TextChanged="cmbFontSize_TextChanged" Width="40" VerticalAlignment="Stretch"/>
                <Separator />
                <Button Name="SaveNote" Content="Сохранить изменения" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
                <Separator />
                <Button Name="DeleteNote" Content="Удалить заметку" Width="100" VerticalAlignment="Stretch"/>
            </ToolBar>
        </DockPanel>
        <TextBox Text="{Binding EditorHeader, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" Grid.Row="1" FontFamily="Times New Roman" FontSize="18" VerticalAlignment="Center"/>
        <RichTextBox x:Name="rtbEditor" Grid.Row="2" DataContext="{Binding EditorTextRange, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"/>
    </Grid>
</UserControl>