Collection<BlockTransformer> blockTransforms = new ArrayList<>();
if (excludeAir) {
  blockTransforms.add((region, x, y, z, current, state) -> {
    BlockState stateInWorld = state.getWorld();
    if (current.getType().isAir() && !stateInWorld.getType().isAir())
      current.setBlockData(stateInWorld.getBlockData().clone());
    return current;
  });
} 

CraftStructureTransformer https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/src/main/java/org/bukkit/craftbukkit/util/CraftStructureTransformer.java#117

Will throw even before the added BlockTransformer can execute. When pasting a structure at a location that isn't it's origin copy location, Spigot will throw an error stating that the coordinates are not within the LimitedRegion `IllegalArgumentException: Coordinates -20, 65, 80 are not in the region` and this is correct. The LimitedRegion doesn't have the origin blockstate coords. The coordinates are different, as it's pasting in a different location.

Just adding an empty Collection<BlockTransformer> will also cause the error to throw.