Recently, the Twenty Nineteen theme had been moved from Github to Trac. As Twenty Nineteen is the first core theme that uses npm and therefore contains a node_modules folder, running
grunt resulted in an error. I just created a minor patch, to solve this issue, and to make WordPress yet a little bit better. 😀
The Trac ticket can be found on Ignore theme based node_modules and my patch can be viewed via 46099.diff.