title: Task :app:uploadCrashlyticsMappingFileRelease FAILED Expected file collection to contain exactly one file, however, it contains no files
body: <p>I have implemented <code>Firebase</code> <code>crashlytics</code> as suggested. I have put this in my app level <code>release</code> build variant:</p>
  <pre><code>firebaseCrashlytics
    {
      mappingFileUploadEnabled true
    }
  </code></pre>
  <p>but when I start to build signed <code>apk</code> I am always getting this error</p>
  <blockquote>
  <p>Task :app:uploadCrashlyticsMappingFileRelease FAILED</p>
  </blockquote>
  <p>FAILURE: Build failed with an exception.</p>
  <ul>
  <li>What went wrong:
  Execution failed for task ':app:uploadCrashlyticsMappingFileRelease'.</li>
  </ul>
  <blockquote>
  <p>Expected file collection to contain exactly one file, however, it contains no files.</p>
  </blockquote>
  <p>If I set <code>mappingFileUploadEnabled</code> to <code>false</code> then release <code>apk</code> builds successfully. I have searched a lot but couldn't find a work around. Anyone can help?</p>