RichTextBox insert text application will crash

* .NET Core Version: netcoreapp 3.1
* Windows version: Windows 10 CMGE
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
* Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No.
* Security issues and bugs should be reported privately, learn more via our [responsible disclosure guidelines](https://github.com/dotnet/wpf/blob/master/README.md#reporting-security-issues-and-security-bugs).

**Problem description:**
the problem occur when we set the caret on the BlockUIContainer and typing any character,that will raise FatalExecutionEngineError in framework 4.7.2 or raise ExecutionEngineException in netcore 3.1.

**How to reproduce?**
1. create an wpf project,and add this xaml code to mainwindow.xaml

``` XAML

```
2. use an image file to instead D:\sample.jpg
3. run app
4.click image and press Arrow-Right
5.type any character