This is because the asset downloading routines haven't been updated to find any assets for 1.2 yet.
My primary recommendation would be to switch to the "stable" branch instead of "default" when you run from source. Default was merged into stable only a few weeks in preparation for 1.1.0, and the changes since are marginal. In stable the download_assets.py script works.
If you want to test something specifically in 1.2.0 (only thing I can think of is the not-yet-perfectly-working makeclothes and fitting stuff), you can download the assets by cloning this repo:
https://github.com/makehumancommunity/o ... assets_11x and then copy everything under base so that it resides under data in the makehuman directory.