Safe way to solve the situation with a metal thermos with a plastic cap: clamp thermos to workbench, pre-drill hole in large sheet of plywood, drill through pre-drilled hole into thermos. Depending on how complete the sublimation is, there may be a boom and you, the plywood, and your drill may potentially get kicked back a meter or two in the worst case, but your plywood shield would likely spare you from any high-velocity plastic fragments that would cause you injury. If you're lucky the pressure is still low enough that it just blows plastic out of the spiral of the drill bit and there is little other than the plastic shavings to work around.
You would definitely want to be wearing earmuffs though.
Aparrently, shooting is the preferred way to disarm potential explosives in such scenarios.