How to incorporate gulp-uncss with yeoman generator gulp-webapp

Question

I included gulp-uncss:

var uncss = require('gulp-uncss');

Then I added its function to the styles task:

gulp.task('styles', () => {
return gulp.src('app/styles/*.scss')
.pipe($.plumber())
.pipe($.sourcemaps.init())
.pipe($.sass.sync({
  outputStyle: 'expanded',
  precision: 10,
  includePaths: ['.']
}).on('error', $.sass.logError))
.pipe($.autoprefixer({browsers: ['last 1 version']}))
.pipe($.sourcemaps.write())
.pipe(uncss({
        html: ['dist/index.html']
    }))
.pipe(gulp.dest('.tmp/styles'))
.pipe(reload({stream: true}));
});

But, it's not working because the index.html hasn't been generated yet.

Given yeoman generator gulp-webapp's gulpfile, how is it possible to make uncss process the main.css file upon build?