Error Can not pass null for argument 'pathString' in child ()

What can be the error of this code?

final DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference();
                mDatabase.child("Empresas " + País).child(Ciudad).child(TipodeEmpresa).addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        usuarios.removeAll(usuarios);
                        for (DataSnapshot snapshot:
                                dataSnapshot.getChildren()) {
                            Usuario usuario = snapshot.getValue(Usuario.class);
                            usuarios.add(usuario);

                        }
                        adapter.notifyDataSetChanged();
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });

This is my logcat

FATAL EXCEPTION: main
                  Process: com.example.matia.tonum, PID: 5582
                  java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
                      at com.google.firebase.database.DatabaseReference.child(Unknown Source)
                      at com.example.matia.tonum.Fragment.ServiceFragment.onCreateView(ServiceFragment.java:120)
                      at android.support.v4.app.Fragment.performCreateView(Fragment.java:2346)
                      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1428)
                      at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1759)
                      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1827)
                      at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:797)
                      at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2596)
                      at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2383)
                      at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2338)
                      at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2245)
                      at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:703)
                      at android.os.Handler.handleCallback(Handler.java:739)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:135)
                      at android.app.ActivityThread.main(ActivityThread.java:5258)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:372)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)

Line 120 refers to @Override

I hope you can help me, thank you very much!