Awesome, nice hint.

pi@raspberrypi:~/MagicMirror $ 
pi@raspberrypi:~/MagicMirror $ npm list | grep electron
├─┬ electron-prebuilt@1.4.5
│ ├─┬ electron-download@3.0.1
npm ERR! peer dep missing: grunt@~0.4.0, required by grunt-yamllint@0.2.0
pi@raspberrypi:~/MagicMirror $

I did this and no more problem now.

npm rebuild --runtime=electron --target=1.4.5 --disturl=https://atom.io/download/atom-shell --abi=50

Thanks much. :)