When you try to delete a conversation tree and then try to edit that character again you will get a error window saying exception occurred..etc continue or exit..
The work around is to manually delete the sub nodes of the conversation topic first, then the parent topic.
The <ParentKey> element was not removed from the child when the parent was deleted.
If you understand XML and module export/import you can fix this by exporting the entire game, removing this element, and re-importing after disabling libraries and starting a NEW game.