Can't delete document on firestore in Android Studio

Everyone I am a new learner of Android Studio, and on our school project, we have three tabs 'album', 'notes', and 'files' and I am getting errors when deleting a document from their subcollection.

For example, this is how we store images on firestore:

private void uploadToFirebase(Uri uri) {
    Log.d("UploadToFirebase", "Starting upload process");
    String caption = uploadCaption.getText().toString();
    final StorageReference imageReference = storageReference.child("Album" + System.currentTimeMillis() + "." + getFileExtension(uri));
    final CollectionReference forAlbum = Utility.getCollectionReferenceForAlbum(courseId);

    imageReference.putFile(uri)
      .addOnSuccessListener(taskSnapshot -> {
        // Get download URL for the uploaded image
        imageReference.getDownloadUrl().addOnSuccessListener(uriResult -> {
          String imageUrl = uriResult.toString();

          DataClass dataClass = new DataClass(imageUrl, caption);

          forAlbum.add(dataClass)
          .addOnSuccessListener(CollectionReference -> {
            progressBar.setVisibility(View.INVISIBLE);
            Toast.makeText(AddtoAlbum.this, "Uploaded", Toast.LENGTH_SHORT).show();
            finish();
          })
          .addOnFailureListener(e -> {
            progressBar.setVisibility(View.INVISIBLE);
            Toast.makeText(AddtoAlbum.this, "Failed to upload image", Toast.LENGTH_SHORT).show();
          });
        });
      })

As you can see above I have two way storing, on firebase storage and firebase firestore. And I have this new Activity to view the image and inside that is a delete option, however every time I hit delete icon on our app it just loads to another page and crashes when I repeat the process. What could possibly the reason of this error?

Here is my way of deleting the image in firebase:

private void deleteImage(String imageUrl) {
  CollectionReference imagesCollectionRef = FirebaseFirestore.getInstance().collection("Albums");
  DocumentReference imageDocRef = imagesCollectionRef.document(imageUrl);

  imageDocRef.delete()
    .addOnSuccessListener(new OnSuccessListener < Void > () {
      @Override
      public void onSuccess(Void aVoid) {
        Toast.makeText(FullImage.this, "Image deleted", Toast.LENGTH_SHORT).show();
        finish();
      }
    })
    .addOnFailureListener(new OnFailureListener() {
      @Override
      public void onFailure(@NonNull Exception e) {
        Toast.makeText(FullImage.this, "Failed to delete image", Toast.LENGTH_SHORT).show();
      }
    });
}

I tried to put logs on it but it doesn't show anything it just loads to previous page. Ive been trying to delete the files on storage instead but I am getting so much error.

This what I got on logcat:

FATAL EXCEPTION: main
Process: com.example.project_acadeamease1, PID: 10281
java.lang.NullPointerException: Provided document path must not be null.
at com.google.firebase.firestore.util.Preconditions.checkNotNull(Preconditions.java: 148)
at com.google.firebase.firestore.CollectionReference.document(CollectionReference.java: 103)
at com.example.project_acadeamease1.Utility.getCollectionReferenceForAlbum(Utility.java: 25)
at com.example.project_acadeamease1.FullImage.deleteImage(FullImage.java: 59)
at com.example.project_acadeamease1.FullImage.access$100(FullImage.java: 20)
at com.example.project_acadeamease1.FullImage$2.onClick(FullImage.java: 51)
at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java: 188)
at android.os.Handler.dispatchMessage(Handler.java: 106)
at android.os.Looper.loop(Looper.java: 246)
at android.app.ActivityThread.main(ActivityThread.java: 8653)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java: 602)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java: 1130)