ref2 = FirebaseDatabase.getInstance().getReference();
    mDatabase = FirebaseDatabase.getInstance().getReference();

                        Query query = ref2.child("place_id").orderByChild("placeID").equalTo(resID);
                        query.addListenerForSingleValueEvent(new ValueEventListener() {
                            @Override
                            public void onDataChange(DataSnapshot dataSnapshot) {
                                if(dataSnapshot.exists()){
                                    RestaurantInformation resInfo_P = dataSnapshot
                                                      .child("place_id")
                                                      .child(resID).getValue(RestaurantInformation.class);

                                    businessID = resInfo_P.getBusinessID(); // null object exception


                                } else {

                                    ...
                                }
                            }

                            @Override
                            public void onCancelled(DatabaseError databaseError) {

                            }
                        });

public class RestaurantInformation {

    private String resName;
    private String status;
    private String businessID;
    private String placeID;

    public RestaurantInformation() {

    }

    public RestaurantInformation(String businessID, String placeID) {
        this.businessID = businessID;
        this.placeID = placeID;
    }

    public RestaurantInformation(String resName) {
        this.resName = resName;

    }

    public String getResName() {
        return resName;
    }

    public void setResName(String resName) {
        this.resName = resName;
    }

    public String getBusinessID() {
        return businessID;
    }

    public void setBusinessID(String placeID) {
        this.businessID = placeID;
    }

    public String getPlaceID() {
        return placeID;
    }

    public void setPlaceID(String placeID) {
        this.placeID = placeID;
    }
}