It's a bug, that Michael first noticed
here.
It seems to turn up here and there with no rhyme or reason or pattern.
You were probably right to hit "cancel" The temporary solution, of course, is to copy anything you may want to edit, and then if you get the blank screen - which
happens to me most commonly when I go from regular "edit" to "advanced" - you can always paste the original back in, and then edit and update that.
It's unrelated, but nevertheless analogous, to what we've all experienced with assorted technologies, going back to at least the 70's. My mother used to work for a newspaper,
and the publisher forced the reporters to convert to computers rather than typewriters c. 1974. And she would tell me how an otherwise mild-mannered, sedate middle-aged man
would randomly explode into a fury of profanity, when someone would walk by his desk, and the friction of the shoes of the passer-by on the newsroom carpet would somehow
generate static electricity, wiping out the reporter's entire article right as the deadline was at hand.