[tilemill] TypeError: Cannot convert null to object
[tilemill] at Function.<anonymous> (/Applications/TileMill.app/Contents/Resources/app/node_modules/tilelive-mapnik/lib/mapnik_backend.js:358:16)
[tilemill] at /Applications/TileMill.app/Contents/Resources/app/node_modules/tilelive-mapnik/lib/lockingcache.js:72:26
[tilemill] at Array.forEach (native)
[tilemill] at LockingCache.<anonymous> (/Applications/TileMill.app/Contents/Resources/app/node_modules/tilelive-mapnik/lib/lockingcache.js:71:23)
[tilemill] at process._tickDomainCallback (node.js:463:13)
I encountered an error: