I consider it a bug, but not a serious one. Checking for op=preview may not be the correct solution because it wouldn't catch problems introduced right before "create" -- but I'm not sure such are caught at all anyway. But this warning should not appear before content is editted.

The solution I would use is to fill in a default template when it is empty but the sig isn't emply. This default template should end in a couple of newlines. The start of it should be small, perhaps "<!-- Type here -->". Someone volunteered to try to do this long ago, but I don't ever expect schedules from volunteers so I'm not surprised nor disappointed that it hasn't been done.