title: I need to update document if exists otherwise create
body: <p>Is there any way to do an update to a document if exists else create a document. Now I am using the <code>update</code> method but if the document does not exist then it will throw an error message.</p>
  
  <pre><code>Firestore.instance.collection('partnerRequests').document(user.uid).updateData( {
                    DateTime.now().millisecondsSinceEpoch.toString():partnerRegistrationFormData})
  </code></pre>
  
  <blockquote>
    <p>Unhandled Exception: type 'PlatformException' is not a subtype of type
    'String'</p>
  </blockquote>