title: Null is not an object (evaluating 'RNFetchBlob.DocumentDir') using Expo
body: <p>I'm using Expo and try to download files using react-native-fetch-blob</p>
  
  <pre><code>import RNFetchBlob from 'rn-fetch-blob';
  
  RNFetchBlob.fetch('GET', 'http://www.example.com/images/img1.png')
    .then((res) =&gt; {
      let status = res.info().status;
    })
    .catch((errorMessage, statusCode) =&gt; {
    })
  </code></pre>
  
  <p>There is an error occurs: <strong>null is not an object (evaluating 'RNFetchBlob.DocumentDir')</strong></p>
  
  <p>What i've already tried:</p>
  
  <pre><code>npm install --save react-native-fetch-blob
  react-native link 
  </code></pre>
  
  <pre><code>npm install --save react-native-fetch-blob
  react-native link react-native-fetch-blob
  </code></pre>
  
  <pre><code>var RNFetchBlob = require('rn-fetch-blob').default
  </code></pre>
  
  <pre><code>RNFB_ANDROID_PERMISSIONS=true react-native link
  </code></pre>
  
  <p>I cannot do <a href="https://github.com/wkh237/react-native-fetch-blob/wiki/Manually-Link-Package" rel="noreferrer">manually linking</a>, because i use expo and do not have /ios and /android folders.</p>
  
  <p>Could anyone help, how to fix this issue? Is <strong>rn-fetch-blob</strong> work with Expo?</p>