Webpack5: Stats handling is broken

[shilman]

Describe the bug

When I run build-storybook in a @storybook/builder-webpack5 project, I get the following error:

/Users/shilman/projects/testing/react-ts/node_modules/@storybook/builder-webpack5/dist/cjs/index.js:182
        stats.toJson(statsOptions).warnings.forEach(function (e) {
                                           ^

TypeError: Cannot read property 'forEach' of undefined
    at /Users/shilman/projects/testing/react-ts/node_modules/@storybook/builder-webpack5/dist/cjs/index.js:182:44

When I log out the stats.toJson results, I get the following:

{
  name: 'preview',
  logging: {
    'webpack.Compiler': { entries: [], filteredEntries: 9, debug: false },
    'webpack.FlagDependencyExportsPlugin': { entries: [], filteredEntries: 4, debug: false },
    'webpack.InnerGraphPlugin': { entries: [], filteredEntries: 1, debug: false },
    'webpack.Compilation': { entries: [], filteredEntries: 23, debug: false },
    'webpack.SideEffectsFlagPlugin': { entries: [], filteredEntries: 1, debug: false },
    'webpack.FlagDependencyUsagePlugin': { entries: [], filteredEntries: 2, debug: false },
    'webpack.buildChunkGraph': { entries: [], filteredEntries: 9, debug: false },
    'webpack.SplitChunksPlugin': { entries: [], filteredEntries: 4, debug: false },
    ModuleConcatenationPlugin: { entries: [], filteredEntries: 7, debug: false },
    'webpack.FileSystemInfo': { entries: [], filteredEntries: 14, debug: false }
  }
}

Needs more investigation, but I will patch this over to allow people to test out webpack5

[shilman]

Gadzooks!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.2.0-alpha.31 containing PR #13975 that references this issue. Upgrade today to the @next NPM tag to try it out!

npx sb upgrade --prerelease

Closing this issue. Please re-open if you think there's still more to do.