ERROR in Cannot read property 'flags' of undefined

[vedran-m]

Describe the Bug

Can't build the starter cli project with ng build after using "builder": "@angular-builders/custom-webpack:browser" as it returns the error:
ERROR in Cannot read property 'flags' of undefined

Minimal Reproduction

1. Created an empty Angular app using latest (9.1.0) CLI following the guilde on https://angular.io/cli SASS was chosen as the style sheet library.
2. Replaced the builder with "builder": "@angular-builders/custom-webpack:browser"
3. Ran ng build

Expected Behavior

Expected the app to build

Screenshots

Environment

Libs
- @angular/core version: 9.1.0
- @angular-devkit/build-angular version: 0.901.0
- @angular-builders/{the name of the builder} version: 9.0.0

For Tooling issues:
- Node version: v10.15.0  
- Platform:  Windows 

Others:

Additional Context

Add any other context about the problem here.

[just-jeb]

Duplicate of #708

[vedran-m]

Tried the beta, fixes the issue.