I am sorry to hear you are facing issues due to a currently not supported functionality. We have an item logged in our backlog to provide this functionality: RichTextBox: Implement add and delete list undoable commands. You can cast your vote for the implementation as well as subscribe to the task by clicking the Follow button so you can receive updates when its status changes.
I hope you find this information helpful. Please, let me know if there is anything else I can assist you with.
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