MVVM access Richtextbox flowdocument from VM

I am trying to make a RichTextBox with a FlowDocument that I can insert text at the caret position. I can add text to the end of the document. I think I am missing something in my setup to that allows my VM to access the Flowocument or I am setting it up wrong. If I create a FlowDocument in my VM and try to set my RichTextBox to it I get an error that my MyEditor (RichTextBox) does not exist. I can add text to the RichTextBox using what I call the AddItemBtn from a ListBox so at least that much works.

My question is "How should I set my RichTextBox/FlowDocument up?

XAML code

<Window x:Class="Scripter.Views.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Scripter.ViewModels"
    xmlns:wpftoolkit="clr-namespace:Xceed.Wpf.Toolkit;assembly=Xceed.Wpf.Toolkit"
    Title="MainWindow" Height="350" Width="725">


<Grid HorizontalAlignment="Stretch">

    <Grid HorizontalAlignment="Stretch" Height="72" Margin="10,14,0,0" VerticalAlignment="Top" Width="auto">
        <WrapPanel HorizontalAlignment="Left" Height="50" Margin="10,0,0,0" VerticalAlignment="Top">


        </WrapPanel>
        <Button x:Name="OpenFilesBtn" Content="Open" HorizontalAlignment="Left" Margin="15,10,0,0" VerticalAlignment="Top" Width="75" Command="{Binding OpenFileBtn}"/>
        <Button x:Name="SavefilesBtn" Content="Save" HorizontalAlignment="Left" Margin="104,10,0,0" VerticalAlignment="Top" Width="75" Command="{Binding SaveFileBtn}"/>
        <TextBlock x:Name="OpenFile" Text="{Binding OpenFile,Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left" Margin="15,37,0,0" VerticalAlignment="Top" Width="353"/>
        <ComboBox x:Name="TipsBtn" SelectedIndex="0" ItemsSource="{Binding Path=Tabs, UpdateSourceTrigger=PropertyChanged}" SelectedItem="{Binding Path=SelectedOption}"  HorizontalAlignment="Left" Margin="538,10,0,0" VerticalAlignment="Top" Width="120"/>
        <Button x:Name="AddItemBtn" Content="Add Item" HorizontalAlignment="Left" Margin="417,10,0,0" VerticalAlignment="Top" Width="100" Command="{Binding AddItemBtn}" CommandParameter="{Binding ElementName=AddItemList,Path=SelectedItem}"/>

    </Grid>
    <Grid Margin="10,100,10,0">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="2*"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <RichTextBox  Grid.Column="0" x:Name="MyEditor" SelectionChanged="MyEditor_SelectionChanged" ScrollViewer.VerticalScrollBarVisibility="Auto" Margin="0" Height="Auto" HorizontalAlignment="Stretch"  VerticalAlignment="Stretch" Width="Auto" IsDocumentEnabled="True" AcceptsTab="True" AcceptsReturn="True" >
            <RichTextBox.Resources>
                <Style TargetType="{x:Type Paragraph}">
                    <Setter Property="Margin" Value="0" ></Setter>
                    <Setter Property="FontSize" Value="15"></Setter>
                </Style>
            </RichTextBox.Resources>
            <FlowDocument >
                <Paragraph >
                    <Run Text="{Binding TestText}" ></Run>
                </Paragraph>
            </FlowDocument>
        </RichTextBox>
        <ListBox x:Name="AddItemList" Grid.Column="1" Width="Auto" Height="Auto" ItemsSource="{Binding Path=OptionsToChoose}" SelectedItem="ItemSelected">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock x:Name="TextSelected" Text="{Binding Description}" />
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

    </Grid>
</Grid>

VM code that has the issue

public ScripterViewModel()
    {
        ScripterModel scripterModel = new ScripterModel();

        ObservableCollection<string> tabsChoice = new ObservableCollection<string>();
        tabsChoice.Add("Tabs");
        tabsChoice.Add("Buttons");
        Tabs = tabsChoice;

        this.OpenFileBtn = new DelegateCommand(chooseFile, canChooseFile).ObservesProperty(() => OpenFile);
        this.SaveFileBtn = new DelegateCommand(saveFile, canSaveFile).ObservesProperty(() => SaveFile);
        this.AddItemBtn = new DelegateCommand<Tabbed>(addItem);

        FlowDocument flowDoc = new FlowDocument();

        Paragraph p = new Paragraph(new Run("new paragraph"));
        flowDoc.Blocks.Add(new Paragraph(new Run("Paragraph 1")));
        flowDoc.Blocks.Add(p);

        //MyEditor = flowDoc;

    }

    public void MyEditor_SelectionChanged(object sender, RoutedEventArgs e)
    {
      // TextRange tempRange = new TextRange(MyEditor.Document.ContentStart, MyEditor.Selection.Start);
        MessageBox.Show("Selection Changed");

    }

    private string _testText;
    public string TestText
    {
        get
        {
            return _testText;
        }

        set
        {
            string _temp;
            _temp = _testText + value;
            SetProperty(ref _testText, value);
        }

    }