Addendum: this entire volatile situation (and how worried Fritz is about it) is also why Fritz permits Heinrich to go to Sweden to talk to sister Ulrike in the first place. Reminder that outside of the war, this was only Heinrich's second time (after the earlier trip to the Netherlands in the late 1760s) abroad, and that he knew he had to make it count because Fritz wouldn't give him permission to go again otherwise any time soon, hence the secret scheming with Catherine so that she writes to Fritz once Heinrich is in Sweden (but not before).