java.lang.NullPointerException：尝试调用虚方法＆＃39; java.util.ArrayList com.example.priyanka.rent_ol.Firebase.Firebase_helper.retrieve（）

Question

我是android编程的新手，我正在从firebase数据库中检索数据到recyclerview但是我得到空指针异常。错误如下。

E/UncaughtException: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.ArrayList com.example.priyanka.rent_ol.Firebase.Firebase_helper.retrieve(java.lang.String)' on a null object reference
                     at com.example.priyanka.rent_ol.Get.getData(Get.java:39)
                     at com.example.priyanka.rent_ol.AllProducts.onClick(AllProducts.java:43)
                     at android.view.View.performClick(View.java:4757)
                     at android.view.View$PerformClick.run(View.java:19757)
                     at android.os.Handler.handleCallback(Handler.java:739)
                     at android.os.Handler.dispatchMessage(Handler.java:95)
                     at android.os.Looper.loop(Looper.java:135)
                     at android.app.ActivityThread.main(ActivityThread.java:5235)
                     at java.lang.reflect.Method.invoke(Native Method)
                     at java.lang.reflect.Method.invoke(Method.java:372)
                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898)
                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)

allproducts类中的Onclick方法如下

@覆盖     public void onClick（查看视图）{

    if(view.getId()==R.id.electronics_button){
        Toast.makeText(AllProducts.this,"electronics clicked",Toast.LENGTH_LONG).show();
        category="Cloths";
        getitem.getData(category);
        startActivity(new Intent(getApplicationContext(),Get.class));
    }

}

Get Data方法如下

public void getData(String category) {
    //SETUP RV
  //  rv= (RecyclerView) findViewById(R.id.recyclerv);
   // rv.setLayoutManager(new LinearLayoutManager(this));
    catS=category;
    adapter = new MyAdapter(this, helper.retrieve(catS));
    rv.setAdapter(adapter);
}

我的Firebase助手类如下

 DatabaseReference AdvertisementRef;
ArrayList<SaveAddInformation>adinfolist=new  ArrayList();
MyAdapter adapter;

public Firebase_helper(DatabaseReference advertisementRef) {
    AdvertisementRef = advertisementRef;
}

//READ DATA

public ArrayList<SaveAddInformation> retrieve(String cat){
    String categoryS;
    categoryS=cat;

    AdvertisementRef.orderByChild("main_category").equalTo(categoryS).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot eventSnapshot : dataSnapshot.getChildren()) {

                SaveAddInformation mModel = eventSnapshot.getValue(SaveAddInformation.class);
                Log.d("DATA" ,""+ mModel);
                adinfolist.add(mModel);
            }
           // adapter.notifyDataSetChanged();
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }

    });

    return adinfolist;

}