Blocks that don't exist as an item show up as "Air" in Superflat customisation GUI

What I expected to happen was...:
The name of the block to appear in the graphical list of block layers, along with either a blank icon or some representation of the texture.

What actually happened was...:
The name and the icon of the block is replaced by "Air"

Steps to Reproduce:
1. Go to Singleplayer → Create new world → More world options → Customise (with Superflat selected as the world type)
2. Enter the following preset code: minecraft:stone,minecraft:nether_portal;minecraft:plains; and select Use Preset.