title: firebase backend function cannot read property
body: <p>hello everyone i am kind of new to android programming and almost have no idea in js coding , i followed a tutorial to create a firebase function and when i did it it seem to have an error , my question may be stupid but i need help , i am getting an error on the second line from the user_id ( i put a ``` just for Illustration : </p>
  
  <pre><code>exports.sendNotification = functions.database.ref('/n/{user_id}/{notification_id}').onWrite(event =&gt; {
  const user_id = event.params.```user_id```;
  const notification_id = event.params.```notification_id```;
  </code></pre>
  
  <p>this is my database :</p>
  
  <pre><code> -n
  
  --ti9MmffcXLT38uagj5J3fTqshjf1
  
  ----LBlr1He51ln-yTyQOdM
  
  --------f: "FjVudM9BBahhCd0eta9lWS8tUoY2"
  
  --------t : 5
  </code></pre>
  
  <p>i am getting this error:</p>
  
  <pre><code>TypeError: Cannot read property 'user_id' of undefined
  at exports.sendNotification.functions.database.ref.onWrite.event (/user_code/index.js:28:31)
  at Object.&lt;anonymous&gt; (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:112:27)
  at next (native)
  at /user_code/node_modules/firebase-functions/lib/cloud-functions.js:28:71
  at __awaiter (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:24:12)
  at cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:82:36)
  at /var/tmp/worker/worker.js:710:26
  at process._tickDomainCallback (internal/process/next_tick.js:135:7)
  </code></pre>
  
  <p>my hands are tied here and i really need to finish this , any help will be appricated :)</p>