It's not happening. Even kim jong un isn't crazy enough to sign his own death warrant.
Really, people just need to understand this. While you can never fully know an adversary, you can make well-evidenced conclusions that a regime like Kim's is merely about survival. It's not going to act irrationally.
Iran and Pakistan are much more worrying to me, because I'm not fully convinced they're rational agents, due to the import of radical Islam in each country. Luckily, Pakistan is a secular regime and Iran, while a theocracy, is most likely more concerned about survival than apocalypse. It would take a regime with the apocalyptic vision of ISIS in Tehran, and, for all its gnashing of teeth about Israel's demise and America's death, it is not an apocalyptic regime. The ayatollahs are actually fairly learned men, who aren't really that concerned with "jihad."
My biggest fears regarding nuclear weapons are the following:
1. Pakistani state failure
2. Saudi-Iranian nuclear proliferation that is unsettled by religious extremism in both regimes
3. Non-state actors getting a weapon, or at least a dirty bomb. (We already know an unknown Russian "marketer" has attempted to sell to terrorists groups no less than three times since the late 90s. That is on official record.)
The only ones who defy rational actor theory are the religious extremists, because they simply don't care. The rules that apply to the rest of us just don't apply to them. This is the reason why the Soviets, as bad as they were, never let the nukes fly. Because they were ultimately rational actors and not religious nutbags who taught they'd be in Heaven no matter what happens. Religious extremists are therefore most likely the only candidates to actually consider using a nuclear weapon preemptively.
I'm not that concerned about North Korea letting the nukes fly or Iran preemptively nuking Israel, although the fact that these novices are not as skilled in nuclear diplomacy will be ample cause for alarm from time to time.