message: Add optional chaining to builder check
diff: @@ -157,7 +157,7 @@ const findConfigFlags = async ({ options, packageJson }) => {
  export const findBuilder = async (mainConfig) => {
  let name = 'webpack4'; // default builder in Storybook v6
  - if (mainConfig?.core.builder) {
  + if (mainConfig?.core?.builder) {
  const { builder } = mainConfig.core;
  name = typeof builder === 'string' ? builder : builder.name;
  }
  