npm run start locks up with 402 error

After cloning and running npm install, I run npm run start. The seems to build, but then stops with the message:

rollup v1.23.0
bundles /Users/thad/polymer/weightless/src/demo/main.ts → dist...
[!] (plugin Typescript) TS2322: Type 'null' is not assignable to type 'string'.
src/lib/behavior/overlay/overlay-behavior.ts (402:5)

402             $container.style.overflow = null;
                ~~~~~~~~~~~~~~~~~~~~~~~~~

and I have to Ctrl-C to get out. I have TypeScript 3.6.3 installed on macOS with Node.js 12.11.1. What more info do you need?

BTW, npm-check-updates errors with unknown option '-a'

andreasbm / weightless

npm run start locks up with 402 error #84