Data path ".builders['app-shell']" should have required property 'class' when i was trying to use firebase with angular i got this error

Question

Schema validation failed with the following errors: Data path ".builders['app-shell']" should have required property 'class'. Error: Schema validation failed with the following errors: Data path ".builders['app-shell']" should have required property 'class'. at MergeMapSubscriber.project (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules@angular-devkit\core\src\workspace\workspace.js:215:42) at MergeMapSubscriber._tryNext (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\operators\mergeMap.js:69:27) at MergeMapSubscriber._next (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\operators\mergeMap.js:59:18) at MergeMapSubscriber.Subscriber.next (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\Subscriber.js:67:18) at MergeMapSubscriber.notifyNext (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\operators\mergeMap.js:92:26) at InnerSubscriber._next (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\InnerSubscriber.js:28:21) at InnerSubscriber.Subscriber.next (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\Subscriber.js:67:18) at MapSubscriber._next (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\operators\map.js:55:26) at MapSubscriber.Subscriber.next (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\Subscriber.js:67:18) at SwitchMapSubscriber.notifyNext (C:\Users\mjha3\Documents\Ang_proj\paper-kit-2-angular-master\node_modules@angular\cli\node_modules\rxjs\internal\operators\switchMap.js:86:26)

this is the error I am getting. My project was fine untill I tried firebase database. I tried my best to solve this but now I need help. Following are something that I have tried so far:

1. I uninstall the "@angular-devkit/build-angular" then install it again
2. I change the version of devkit from package.json
3. I updated @angular/cli

still, I am getting the same error.