如何检查firebase文档是否存在？

Question

我正在创建一个用于预订的应用。 在预订历史页面，如果用户还没有预订，应该是这样的：

但是当我执行它时，它是这样的：

FireStore 数据库图像：

代码：

 DocumentReference documentReference = fstore.collection("Booking").document(userID);
    //if the document exists then set text and make visible, but if it doesn't exist dont be visible
    //i am a beginner i set this if condition it doesn't seem to work it just sets the textviews to visible please help.
    if(documentReference!=null) {
        documentReference.addSnapshotListener(this, new EventListener<DocumentSnapshot>() {
            @Override
            public void onEvent(@Nullable DocumentSnapshot documentSnapshot, @Nullable FirebaseFirestoreException e) {
                //putting if else fixed crashing
                if (e != null) {
                    Log.d("Tag", "Error:" + e.getMessage());
                } else {
                    //pickup
                    mYourpickup.setVisibility(View.VISIBLE);
                    mYourdestination.setVisibility(View.VISIBLE);
                    mBook.setVisibility(View.VISIBLE);
                    mFirstnamelastname.setText(documentSnapshot.getString("a1 - Fullname"));
                    mFirstnamelastname.setVisibility(View.VISIBLE);
                    mMobnum.setText(documentSnapshot.getString("a2 - PhoneNo"));
                    mMobnum.setVisibility(View.VISIBLE);
                    mPincode.setText(documentSnapshot.getString("a3 - Pincode"));
                    mPincode.setVisibility(View.VISIBLE);
                    mFlatno.setText(documentSnapshot.getString("a4 - Flatno"));
                    mFlatno.setVisibility(View.VISIBLE);
                    mArea.setText(documentSnapshot.getString("a5 - Area"));
                    mArea.setVisibility(View.VISIBLE);
                    mLandmark.setText(documentSnapshot.getString("a6 - Landmark"));
                    mLandmark.setVisibility(View.VISIBLE);
                    mTown.setText(documentSnapshot.getString("a7 - Town"));
                    mTown.setVisibility(View.VISIBLE);
                    mState.setText(documentSnapshot.getString("a8 - State"));
                    mState.setVisibility(View.VISIBLE);
                    //destination
                    mFirstnamelastname2.setText(documentSnapshot.getString("d1 - Fullname2"));
                    mFirstnamelastname2.setVisibility(View.VISIBLE);
                    mMobnum2.setText(documentSnapshot.getString("d2 - PhoneNo2"));
                    mMobnum2.setVisibility(View.VISIBLE);
                    mPincode2.setText(documentSnapshot.getString("d3 - Pincode2"));
                    mPincode2.setVisibility(View.VISIBLE);
                    mFlatno2.setText(documentSnapshot.getString("d4 - Flatno2"));
                    mFlatno2.setVisibility(View.VISIBLE);
                    mArea2.setText(documentSnapshot.getString("d5 - Area"));
                    mArea2.setVisibility(View.VISIBLE);
                    mLandmark2.setText(documentSnapshot.getString("d6 - Landmark2"));
                    mLandmark2.setVisibility(View.VISIBLE);
                    mTown2.setText(documentSnapshot.getString("d7 - Town2"));
                    mTown2.setVisibility(View.VISIBLE);
                    mState2.setText(documentSnapshot.getString("d8 - State2"));
                    mState2.setVisibility(View.VISIBLE);
                }

            }
        });
    }

请在您回答此问题时尝试向我解释发生了什么，以便我学习和理解而不是复制粘贴。 谢谢:)