java.lang.NullPointerException: Attempt to invoke virtual method [duplicate]

My program gave this problem when I tried to create a stack (database), to store strings in my Android app

package com.project.meuapp2;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;


public class Principal extends AppCompatActivity {
    private Button btn;
    private EditText texto;
    BancodeDados DB;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_principal);
        texto = (EditText) findViewById(R.id.texto);
        btn = (Button) findViewById(R.id.btn);
        texto.setText("Insira um dado");
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String y = texto.getText().toString();
                DB.InsereInicio(y);
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main, menu);
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected (MenuItem item){
        Intent i = null;

        switch(item.getItemId()) {
            case R.id.mnTela1:
                i = new Intent(this, pagina2.class);
                Toast.makeText(this, "item1", Toast.LENGTH_LONG).show();
                startActivity(i);
                break;
            case R.id.mnTela2:
                Toast.makeText(this, "item2", Toast.LENGTH_LONG).show();
                break;
        }
        return super.onOptionsItemSelected(item);
    }
}

package com.project.meuapp2;

public class BancodeDados {
    elemento primeiro = new elemento();
    elemento ultimo = new elemento();

    public void InsereInicio(String x){
        elemento novodadoinicio = new elemento();
        novodadoinicio.dado = x;
        novodadoinicio.proximo = primeiro;
        if(primeiro==null){
            ultimo = novodadoinicio;
        }
        primeiro = novodadoinicio;
    }

    public void InsereFinal(String x){
        elemento novodadoultimo = new elemento();
        novodadoultimo.dado = x;
        novodadoultimo.proximo = primeiro;
        if(ultimo==null){
            primeiro = novodadoultimo;
        }
        ultimo = novodadoultimo;
    }

    public String removeInicio(){
        String x;
        x = primeiro.dado;
        primeiro = primeiro.proximo;
        return x;
    }
}

Android Studio Log

  

03-27 18: 14: 28,017 5239-5239 / com.project.meuapp2 E / AndroidRuntime:   FATAL EXCEPTION: main                                                                      Process: com.project.meuapp2, PID: 5239                                                                      java.lang.NullPointerException: Attempt to invoke virtual method 'void   com.project.meuapp2.InstanceInstance (java.lang.String) 'on a   null object reference                                                                          at com.project.meuapp2.Principal $ 1.onClick (Main.java:30)                                                                          at android.view.View.performClick (View.java:5198)                                                                          at android.view.View $ PerformClick.run (View.java:21147)                                                                          at android.os.Handler.handleCallback (Handler.java:739)                                                                          at android.os.Handler.dispatchMessage (Handler.java:95)                                                                          at android.os.Looper.loop (Looper.java:148)                                                                          at android.app.ActivityThread.main (ActivityThread.java:5417)                                                                          at java.lang.reflect.Method.invoke (Native Method)                                                                          at   com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run (ZygoteInit.java:726)                                                                          at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:616)

Note: I have tried to initialize the variable primeiro , but it has the same error.