A good warhead design does not go critical from a single point initiation of the explosives.
NO warhead design does.
If it was so easy to make plutonium go critical, that you could do so
by mistake, the Manhattan Project would have lasted a couple of months and cost a few thousand dollars.
The ONLY way to set of a nuke by mistake would be somehow to actuate the triggering mechanism by mistake, on an intact, undamaged, and fully armed warhead.
Preventing this from happening accidentally is trivially easy. The hard part is getting the damn thing to go critical when you actually want it to, which is why the Manhattan Project took so long that the war in Europe was over before they had a bomb; Cost more than any prior weapons reseach program (buy FAR); And still had to waste a large fraction of the world's entire supply of hugely expensive Pu on a test firing, because the smartest men on Earth weren't sure that it could be done - the timings on the conventional explosives had to be accurate to the nanosecond, which at the time was a higher precision than had ever been attempted.
If you could design a nuke that could be accidentally triggered, without using it's own intact and fully functional triggering mechanism, the world would be a very different place (and Berlin would have been a radioactive crater in 1944).