Current user uid is returning as null

I'm trying to send data with the current user uid to Firebase Firestore but It is showing -> TypeError: Cannot read property 'uid' of null.

I'm currently logged in and authenticated.

This is the code for sending the data

 db.collection("news").add({
     newsImage: url,
     newsTitle: title,
      newsContent: content,
      newsPublisher: publisher,
      timestamp: firebase.firestore.FieldValue.serverTimestamp(),
      adminUid: user.uid
      }).then(function (docRef) {
           console.log("Document written with ID: ", docRef.id);
      }).catch(function (error) {
           console.error("Error adding document: ", error);
   });

This is my firestore rules

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write: if false;
    }
  }
}