When opening the attached Fireworks prefab in Prefab Mode the console gets filled with:
'Sub-emitters must be children of the system that spawns them'.

Repro on a 2018.2/staging based branch
1) Import the attached prefab: Fireworks.prefab into a empty project
2) Drag it to the Hierarchy
3) Put a break point in ParticleSystem::CollectSubEmittersRecursive and inspect the results of this function.
4) Note that 8 emitters are collected but there are only 6 active emitters

Repro of the console logging:
Note to get the spamming in the console you need to pull the branch '2018.2/prefabs/improved' and then double clicking the Fireworks.prefab in the Project Browser

See screenshots below