With Full Slabs and Mishang Urban Construction installed, the air mask in WorldEdit no longer works. No matter what, it will not find any air in your selected area.

In this screenshot, the selection area is visible using WorldEditCUI, and I've just issued the command //replace #air sand

This is only an issue when both Full Slabs and Mishang Urban Construction are present. Full Slabs does not break the #air mask by itself, nor does Mishang Urban Construction by itself.

Here is a log with both mods loaded where I attempted to use the #air mask: https://mclo.gs/BgZhDjL
I've also submitted an issue to Full Slab's GitHub.