when ApplicationBase's validate_app is called, it tries to create the casexml. Here, in the creation fn, I check that each subcase exists in the app, and if it raises an error, the error gets captured by validate_app. (https://github.com/dimagi/commcare-hq/pull/2064)

when viewing the problematic form, this line is called: https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/app_manager/views.py#L343

3 lines below it, it catches CaseErrors (which will get raised when creating casexml and the subcase doesn't exist) and displays the alert for the message