generator fails because .gitignore does not exist

I tried running the generator got the following:

events.js:182
      throw er; // Unhandled 'error' event
      ^

AssertionError [ERR_ASSERTION]: Trying to copy from a source that does not exist: /usr/local/share/.config/yarn/global/node_modules/generator-buildbot-dashboard/generators/app/templates/.gitignore
    at EditionInterface.exports.copy (/usr/local/share/.config/yarn/global/node_modules/mem-fs-editor/lib/actions/copy.js:47:3)
    at EditionInterface.module.exports [as copyTpl] (/usr/local/share/.config/yarn/global/node_modules/mem-fs-editor/lib/actions/copy-tpl.js:10:8)
    at module.exports.writing (/usr/local/share/.config/yarn/global/node_modules/generator-buildbot-dashboard/generators/app/index.js:115:21)
    at Object.<anonymous> (/usr/local/share/.config/yarn/global/node_modules/yeoman-generator/lib/index.js:417:23)
    at /usr/local/share/.config/yarn/global/node_modules/run-async/index.js:25:25
    at Promise (<anonymous>)
    at /usr/local/share/.config/yarn/global/node_modules/run-async/index.js:24:19
    at /usr/local/share/.config/yarn/global/node_modules/yeoman-generator/lib/index.js:418:9
    at runCallback (timers.js:781:20)
    at tryOnImmediate (timers.js:743:5)

I was able to get past this by manually creating an empty .gitignore in the directory where it was looking for it.

Inspect generators/app/index.js around line 115 and the generators/app/templates directory, focusing on the missing .gitignore source identified in the error. Re-run the generator after the change; done means it no longer raises the missing-source assertion.