The issue is related to specific steps and is a regression released in R3 2018.

Steps to reproduce:
1. Run RadRichTextBox and insert "test" at the beginning of the document.
2. Move the caret after the 'e'
3. Press Shift+Enter to insert a line break

Observed: NullReferenceException is thrown and the UI becomes unresponsive.
Expected: The break should be successfully inserted in the content