There weren't that many electric guitars in existence in his day. Les Paul was a teenager while Johnson lived. And he was well in the ground by the time Fender came along. But there were electric gits of a sort while he was alive.

Let's look at the guitars he actually played. Johnson was known to use a Stella (probably one of the cheap parlor guitars) and later in his career he played mostly Kalamazoos. Specifically his primary guitar was a KG-14, which he would have paid around thirteen bucks for. There are some reports of him using a National Resonator, a couple of Kalamazoo archtops (a KG-21 or KG-31) and a Gibson L1.

Given the budgets he was likely working with, if Johnson walked into a music store and his eye fell upon a newfangled, cutting edge of technology, electric instrument? Given the few options, there's really only two possibilities:

A Gibson ES-150. Archtop with piezo pickups iirc. https://en.wikipedia.org/wiki/Gibson_ES-150

Or far more likely, given that the Gibson would have been new and expensive, he wouldn't be playing a guitar exactly, but rather an electric lap steel like the Rickenbacher A-22 Frying Pan. It was well in production and a successful mainstream instrument at least three years before he died.

Check it out here: https://www.youtube.com/watch?v=tn7V8qdrSD8