Just did an clean install of the released version of Lightroom 4 and I am also experiencing the edit-in error "Unexpected error performing command:bad argument #1 to 'lower' (string expected, got nil)". I have the complete Nik suite of external editors and did a fresh install of them also under Windows 7 64-bit. This all worked will under LR 3.6.
Adobe support is useless in this as they say they don't support external 3rd party editors... Hope the community can help!
Europe, Middle East and Africa