java.lang.NullPointerException: Provided document path must not be null.
        at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:906)
        at com.google.firebase.firestore.CollectionReference.document(com.google.firebase:firebase-firestore@@18.2.0:110)
        at com.example.home.optometryapplication.AdminUpdateActivity.updateBook(AdminUpdateActivity.java:155)
        at com.example.home.optometryapplication.AdminUpdateActivity.onOptionsItemSelected(AdminUpdateActivity.java:101)
        at android.app.Activity.onMenuItemSelected(Activity.java:3204)
        at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:407)
        at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:195)
        at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:108)
        at android.support.v7.app.AppCompatDelegateImplV9.onMenuItemSelected(AppCompatDelegateImplV9.java:674)
        at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
        at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171)
        at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973)
        at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:963)
        at android.support.v7.widget.ActionMenuView.invokeItem(ActionMenuView.java:624)
        at android.support.v7.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:150)
        at android.view.View.performClick(View.java:5610)
        at android.view.View$PerformClick.run(View.java:22265)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)

private void updateBook() {

        FirebaseFirestore db = FirebaseFirestore.getInstance();

        String id = getIntent().getStringExtra("id");
        String chapterName = editTextChapterName.getText().toString().trim();
        String chapterInfo = editTextChapterInfo.getText().toString().trim();

        if (chapterName.trim().isEmpty() || chapterInfo.trim().isEmpty()) { //ensure that user has not left boxes empty
            Toast.makeText(this, "Please add a chapter name and the chapter information", Toast.LENGTH_SHORT).show();
            return;
        }

        DocumentReference bookRef = db
                .collection("Book")
                .document(id);
        bookRef
                .update("chapterName", "chapterInfo")
                .addOnSuccessListener(new OnSuccessListener<Void>() {
                    @Override
                    public void onSuccess(Void aVoid) {
                        Log.d(TAG, "DocumentSnapshot updated!");
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Log.w(TAG, "Error", e);
                    }
                });

public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.save_icon:
                updateBook();
                Intent intent = new Intent(AdminUpdateActivity.this, AdminReadActivity.class);
                startActivity(intent);
            return true;
            default:
                return super.onOptionsItemSelected(item);
        }

    }

adapter.setOnItemClickListener(new BookAdapter.OnItemClickListener() {

            public void onItemClick(DocumentSnapshot documentSnapshot, int position) {
                Book book = documentSnapshot.toObject(Book.class);
                String id = documentSnapshot.getId();
                String path = documentSnapshot.getReference().getPath();
                Toast.makeText(AdminReadActivity.this,
                        "Position: " + position + " ID: " + id, Toast.LENGTH_SHORT).show();

                String chapterName = adapter.getItem(position).getChapterName();
                String chapterInfo = adapter.getItem(position).getChapterInfo();
                Integer chapterNumber = adapter.getItem(position).getChapterNumber();


                Intent intent = new Intent(AdminReadActivity.this, AdminUpdateActivity.class);

                intent.putExtra("mChapterName", chapterName);
                intent.putExtra("mChapterInfo", chapterInfo);
                intent.putExtra("mChapterNumber", chapterNumber);
                intent.putExtra("mMyId", id);

                startActivity(intent);