"dimensions":[
{
  "entityId": "minecraft:zombie",
  "dimension": "minecraft:overworld",
  "spawn_chance": 1.0
},
{
  "entityId": "!alexsmobs:.*",
  "dimension": "twilightforest:twilight_forest",
  "spawn_chance": 1.0
},
{
  "entityId": "!alexsmobs:.*",
  "dimension": "deeperdarker:otherside",
  "spawn_chance": 1.0
}
]

Error: java.lang.NullPointerException: Cannot invoke "elocindev.ysns.api.DimensionSpawnHolder.getEntityIdentifier()" because "holder" is null

This cause a crash saying entity register cannot find entity id, though specifying specific entities seem to work just fine with various mods.

Update: The problem seems to be related to modded dimension. Does the corresponding dimension need to exist first?