Description:
Aether Legacy adds its own tab in the inventory, but clicking this icon crashes the client:

The reason this can't currently be fixed:
Even though Aether Legacy (or Aether I) has received an update that fixes this, Tinker's Aether is incompatible with the newer Aether releases. Once Tinker's Aether gets updated, this bug will be fixed.

Workaround:
Don't click the button.

Tinker's Aether has received a port that lets it be used with the newest Aether version: https://www.curseforge.com/minecraft/mc-mods/tinkers-aether-dman-edition
I have updated both mods, which fixes this issue.