Once you get nukes, you are safe from attack. Why? Simple. Even if your delivery system sucks, a low probability of it working is too high a risk to take for attacking you and risking a successful retaliation. Their latest missile test failed. Again. Make a juvenile joke about the names of their missiles. Go on. (Look it up, if you have to). That doesn't mean we can count on all of their missiles failing all of the time, and that means we still can't risk attacking them. Even if we could count on their long-range missiles failing, we have allies nearby. Why did North Korea put up with sanctions for so long to develop nuclear weapons? Because they knew that once they got 'em, they'd be safe. And they are. Trump is the dumbest motherfucker (or wannabe-daughterfucker) to ever get his tiny, tiny hands on nuclear weapons, and even he isn't dumb enough to launch an unprovoked first strike on a nuclear-armed country, and Kim Jong Un certainly isn't dumb enough to provoke anything.
This sabre-rattling? It happens every few years. It sounds worse right now because of Trump, and because he just shot some missiles at Syria, and dropped a MOAB on Afghanistan, but that's about it.
However, with very little information, it is worth saying this: the failed missile test puts Kim's regime in danger from within. If his capacity to deter attacks is questioned by those within North Korea, then he loses his support structure, and those missile tests haven't gone well lately. So, OK, here's a real danger. A country with a couple of nukes might fall into chaos if the psycho dictator loses control of his generals because failed missile tests convince them that he can't deter attacks.
Of course, that's just raw speculation. We don't really know enough to study the internal dynamics that might lead to a coup. Still, go read some recent works by Bruce Bueno de Mesquita, if you are curious about this type of problem.