InvalidOperationException when trying to remove image manually from RichTextBox in WPF

I am trying to create an application where I can enter text and images into a RichTextBox, serialize it and then deserialize it and load it back into the RichTextBox so I can change it later. When I load the images from the serialized xml file everything is displayed correctly, but when I try to remove the image from the RichTextBox manually by pressing backspace I get the following exception: Cannot serialize the non-public type "System.Windows.Media" .Imaging.BitmapFrameDecode ".

Here I am retrieving and saving data from the RichTextBox. It checks all the blocks, and if it finds an image, it just stores the placeholder string in the "List" text, so when it returns, it will know how to return the image to that location:

public void GetFindingsData(FlowDocument flowDoc, List<string> text, List<byte[]> bytes)
    {
        foreach (Block block in flowDoc.Blocks)
        {
            if (block.GetType() == typeof(Paragraph))
            {
                foreach (Run run in ((Paragraph)block).Inlines)
                {
                    text.Add(run.Text);
                }
            }

            else if (block.GetType() == typeof(BlockUIContainer) && ((BlockUIContainer)block).Child.GetType() == typeof(Image))
            {
                Image img = (Image)((BlockUIContainer)block).Child;
                bytes.Add(Storage.ImageToByteArray(img));
                text.Add("imageplaceholder_" + (bytes.Count - 1).ToString());
            }
        }
    }

      

        
        
        
      

    

And this is how I returned this data to the FlowDocument for display in the RichTextBox:

public FlowDocument createFlowDocument(List<string> runs, List<byte[]> bytes)
    {
        FlowDocument flowDoc = new FlowDocument();
        int counter = 0;
        foreach (string run in runs)
        {
            if (run == "imageplaceholder_" + counter.ToString())
            {
                flowDoc.Blocks.Add(new BlockUIContainer(Storage.ByteArrayToImage(bytes[counter])));
                counter++;
            }

            else
            {
                Paragraph par = new Paragraph();
                par.Inlines.Add(run);
                flowDoc.Blocks.Add(par);
            }
        }

        return flowDoc;
    }

      

        
        
        
      

    

If you need it, this is how I serialize the data from the RichTextBox. All my other data is serialized to xml, but that doesn't work for images, so I serialize it to a byte array first:

public static byte[] ImageToByteArray(Image image)
    {
        byte[] imageBuffer = null;

        if (image != null)
        {
            using (var stream = new MemoryStream())
            {
                var encoder = new PngBitmapEncoder();
                encoder.Frames.Add(BitmapFrame.Create(image.Source as BitmapSource));
                encoder.Save(stream);
                imageBuffer = stream.ToArray();
            }
        }

        return imageBuffer;
    }

      

        
        
        
      

    

Here's how I'm serializing and deserializing everything to / from the xml file (although I don't think that's the problem here):

public static void SaveData(StoredData data)
    {
            XmlSerializer serializer = new XmlSerializer(typeof(StoredData));
            using (FileStream stream = new FileStream(readerString, FileMode.Create, FileAccess.Write, FileShare.None))
            {
                serializer.Serialize(stream, data);
            }
    }

public static StoredData LoadData()
    {
        try
        {
            StoredData storedData = new StoredData();
            using (FileStream stream = new FileStream(readerString, FileMode.Open))
            {
                XmlSerializer deserializer = new XmlSerializer(typeof(StoredData));
                storedData = (StoredData)deserializer.Deserialize(stream);
            }
            return storedData;

        }

        catch
        {
            StoredData newData = new StoredData();

            XmlSerializer serializer = new XmlSerializer(typeof (StoredData));
            using (FileStream stream = new FileStream(readerString, FileMode.Create, FileAccess.Write, FileShare.None))
            {
                serializer.Serialize(stream, newData);
            }

            return newData;
        }
    }

      

        
        
        
      

    

And this is how I return an image from a byte array:

public static Image ByteArrayToImage(Byte[] imageBytes)
    {
        using (MemoryStream stream = new MemoryStream(imageBytes))
        {
            BitmapDecoder decoder = BitmapDecoder.Create(stream, BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.OnLoad);

            BitmapFrame frame = decoder.Frames.First();

            frame.Freeze();
            Image newImage = new Image();
            newImage.Source = frame;
            return newImage;
        }
    }

      

        
        
        
      

    

Any help would be greatly appreciated.