“.builders['browser']” should have required property 'class'

In my case, it was a dependency version problem.
I changed the following dependencies version from:

"@angular-builders/custom-webpack": "latest"
"@angular-builders/dev-server": "latest"

to:

"@angular-builders/custom-webpack": "~7.4.3"
"@angular-builders/dev-server": "~7.3.1"

I came across the same error. But in my case I needed to use "@angular-builders/custom-webpack:dev-server" builder, which seems to be only available for Angular CLI 8+. I.e. I have to upgrade to Angular 8 first.

Your case should be simpler, as you only use "custom-webpack:browser", which is available in version 7

So try the following: change versions of "@angular-builders/custom-webpack" and "@angular-builders/dev-server" in package.json to ^7.3.0 (or higher, but not version 8), run npm install or yarn and try building again.

Source: https://github.com/manfredsteyer/ngx-build-plus/issues/109