title: Notify nearby users in cloud functions using geofirestore
body: <p>I have 2 firestore collections - <code>crews/{crew}/clients</code> and <code>crews/{crew}/pros</code>. If a new client registers and a new document is created, I want to search collection <code>pros</code> for pros working the matching sector and living within 5 km (of the new client), and send notification to the pros filtered. In order to implement that in cloud functions,
  I installed geofirestore using npm, saved <code>crews/{crew}/pros</code> like this;
  <a href="https://i.stack.imgur.com/YwAFO.png" rel="nofollow noreferrer">https://i.stack.imgur.com/YwAFO.png</a></p>
  <p>but after executing this function, I have error message on cloud functions console like this;</p>
  <p><code>Error: Registration token(s) provided to sendToDevice() must be a non-empty string or a non-empty array</code></p>
  <p>Is there anything wrong with my firestore data structure? Thank you.</p>