I think I figured this out after trying to run it locally,
The Heroku install steps say to add the follow dependencies to you package.json:
But looking closer, nodebb-plugin-dbsearch is already in the package.json at a higher version. This conflict seemed to cause the crash.
I am guessing that recent versions of nodeBB have changed so this part of the doc is no longer applicable. Here is a pull request to adjust the docs, if I am correct in this assumption: