title: Cannot read property 'apply' of undefined in Webpack Tapable
body: <p>I have recently upgraded from Webpack v1 to v2 and I am getting following error :</p>
  
  <pre><code>TypeError: Cannot read property 'apply' of undefined
  at Compiler.apply (C:\sample\node_modules\Tapable.js:375:16)
  </code></pre>