我如何在Android的firebasefirestore中获取文档ID

Question

我尝试在注册时使用firebase自动生成的ID来设置片段，但它向我显示了null或如何获取ID ？???

documentReference=FirebaseFirestore.getInstance().collection("users").document();

if (task.getResult().size() == 0) {
                    //Toast.makeText(SignUpActivity.this, "Username not exists", Toast.LENGTH_SHORT).show();
                    reference.document(email).set(map).addOnCompleteListener(new OnCompleteListener<Void>() {
                        @Override
                        public void onComplete(@NonNull Task<Void> task) {

                            if (task.isSuccessful()){
                                mProgress.dismiss();
                                 Intent homeActivity=new Intent(SignUpActivity.this, MainActivity.class);
                                 homeActivity.putExtra("userid",email);
                                 homeActivity.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
                                 startActivity(homeActivity);

                                Toast.makeText(SignUpActivity.this, "Register sucessfully", Toast.LENGTH_SHORT).show();
                            }
                            else {
                                Toast.makeText(SignUpActivity.this, ""+task.getException().getLocalizedMessage(), Toast.LENGTH_SHORT).show();

                            }

告诉我零点。