FYI to all:
After testing the runtime issue is still there.
1. RichTextBox is added to the UI
2. Run line Application.Current.Resources.MergedDictionaries.Clear()
3. RichTextBox is added to the UI *Crash*
This does not occur in 2017.1.222, but it does in 2017.2.503 through 2017.2.703
As a workaround until the fix is available, you can merge the required resources before initializing RadRichTextBox after Application.Current.Resources.MergedDictionaries are cleared: