amplify publishでerror: uncaughtException: Cannot read property 'toString' of nullというエラーが出ました

Question

下記URLの教材のSTEP6を進めていたらエラーが出てしましました。

JavascriptでAPIを叩いたことがないからか、あまり理解ができないのでお力をお借りしたいです。

https://feature-ai-service.dma9ecr5ksxts.amplifyapp.com/amplify/amplify-auth/

amplify runやamplify serveでは正常に動いたのにも疑問があります。

Building for production...2020-02-07T07:52:42.621Z - error: 
uncaughtException: Cannot read property 'toString' 
of null date=Fri Feb 07 2020 07:52:42 GMT+0000 (GMT), 
pid=2228, uid=1000, gid=1000, cwd=/home/ec2-user/myamplifyproject,
execPath=/usr/local/bin/node, version=v12.15.0, argv=[/usr/local/bin/node, 
/usr/bin/amplify, publish], rss=185823232, heapTotal=159039488, 
heapUsed=152648776, external=3012001, loadavg=[1.09814453125, 0.59033203125,
 0.40869140625], uptime=8448, trace=[column=74, 
file=/usr/lib/node_modules/@aws-amplify/cli/node_modules/amplify-frontend-javascript/lib/builder.js,
 function=null, line=17, method=null, native=false, column=5, file=events.js, function=ChildProcess.emit, line=223, method=emit, native=false, column=20, file=domain.js,
function=ChildProcess.EventEmitter.emit, line=475, method=emit, native=false, column=12, file=internal/child_process.js,
function=Process.ChildProcess._handle.onexit, line=272, method=onexit,
native=false], stack=[TypeError: Cannot read property 'toString' of null,
     at ChildProcess.<anonymous> (/usr/lib/node_modules/@aws-amplify/cli/node_modules/amplify-frontend-javascript/lib/builder.js:17:74),
     at ChildProcess.emit (events.js:223:5),
     at ChildProcess.EventEmitter.emit (domain.js:475:20),
     at Process.ChildProcess._handle.onexit (internal/child_process.js:272:12)]