When editing a comment, if it has been deleted while the comment author is editing it, the edited text will completely disappear into the void without any sort of warning as soon as the user clicks save, which can lead to confusion. The user should have some notice that their comment will disappear once they save their edit, and given the chance to copy out the content (and reconsider, if appropriate).
Comments are transient and subject to deletion, typically after they have served some purpose. However, disappearing without notice as soon as you click "save" is quite discouraging of investing any time/energy in trying to make a comment that is useful to anybody. It's confusing at best.
There is precedent in the SE system for providing a warning when content that a user is currently editing will disappear. When a user ("A") is editing a post, if another user ("B") submits an edit, A will get a warning that another user has edited the post and that A's edits might be discarded (unless A's version is "more thorough," whatever that means in the system's operationalization). This can prompt A to copy out text they've changed, look at what's been modified in the post, and (especially upon seeing that B's edits were to a different part), resume editing in the revised version. Similarly, if A is editing a post that gets deleted while they're editing it, they'll be prompted of this fact, and the system won't let them submit their edit. Comments are both more numerous and more transient than posts, and having a warning when editing deleted ones would be helpfully consistent.
Additionally, when the 5-minute edit window expires while a comment author is editing the comment, when the user clicks "Save Edits," there is a red warning box that pops up saying "Comments may only be edited for 5 minutes" followed by an X button that will close the message. The edit comment box stays open indefinitely and the user can copy out the edited version of the comment to wherever they want. No matter how many times they click "Save Edits," they'll get that message or if clicking again in short succession, a transient punctuated message in an otherwise similarly-styled box saying "You may only edit a comment every 5 seconds." The edit is not saved but does not just disappear into the void without warning. A similar implementation would be appropriate here, probably persistent-until-close like the first and punctuated like the second. Draft text: "Your edit cannot be saved because this comment has been deleted."
Something like this has previously been requested at Show me a message if a comment I'm editing gets deleted and declined. However, in that post, the premise for the request was about the automatically generated "Possible duplicate of" comment and the fact that it's deleted once the post has been closed as duplicate, possibly while the original voter/flagger is editing the automatic comment. The only/accepted answer and decline reason there only considered this case, and did not consider other cases where a comment may be deleted while it's being edited (e.g. independently posted comments). The only/accepted answer to that question recommended an alternative course of action which, if followed, would still leave this same issue outstanding. The answer there does not answer this question.
Also, the system has been changed to no longer automatically delete that type of automatically generated comment if it's edited, so the main arguments for declining the other request - that "there's a good chance that [the] comment will still get deleted when the question is closed" and that the user is "[depending] on implementation details" - are no longer correct.