Facebook Logout Button SDK 4+. Error uploading activity

When trying to implement facebook logout, in another activity , other than the one that logs in, the following message appears:

Error

  

MainActivity}: java.lang.NullPointerException: Attempt to invoke   virtual method 'void   android.widget.Button.setOnClickListener (android.view.View $ OnClickListener) '   on a null object reference

I used the native Facebook button, with the implementations of the documentation. How can I log out? I put LoginManager.getInstance().logOut(); inside the event of a% common button%.

Main:

public class MainActivity extends AppCompatActivity{
    private Button button;
    private LoginActivity login;
    private Context context;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        FacebookSdk.sdkInitialize(getApplicationContext());
        setContentView(R.layout.activity_main);

        // Solicita as permissões
        String[] permissoes = new String[]{
                Manifest.permission.READ_EXTERNAL_STORAGE,
                Manifest.permission.ACCESS_COARSE_LOCATION,
                Manifest.permission.ACCESS_FINE_LOCATION,
                Manifest.permission.SEND_SMS,
                Manifest.permission.RECEIVE_SMS,
                Manifest.permission.INTERNET,
        };
        Permissions.validate(this, 0, permissoes);
        button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });

        LoginButton loginbutton = (LoginButton)findViewById(R.id.login_button);
        loginbutton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                LoginManager.getInstance().logOut();
            }
        });

    }
}

Login: ...

    callbackManager = CallbackManager.Factory.create();
   // Button fb = (Button) findViewById(R.id.fb);
    LoginButton loginButton = (LoginButton) findViewById(R.id.login_button);
    //assert loginButton != null;
    loginButton.setReadPermissions("public_profile", "email", "user_friends", "user_birthday");
    loginButton.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
        @Override
        public void onSuccess(LoginResult loginResult) {
            String accessToken = loginResult.getAccessToken()
                    .getToken();
            Log.i("accessToken", accessToken);

            GraphRequest request = GraphRequest.newMeRequest(
                    loginResult.getAccessToken(),
                    new GraphRequest.GraphJSONObjectCallback() {@Override
                                                                public void onCompleted(JSONObject object,
                                                                                        GraphResponse response) {

                        Log.i("LoginActivity",
                                response.toString());
                        try {
                            id = object.getString("id");
                            try {
                                URL profile_pic = new URL(
                                        "http://graph.facebook.com/" + id + "/picture?type=large");
                                Log.i("profile_pic",
                                        profile_pic + "");

                            } catch (MalformedURLException e) {
                                e.printStackTrace();
                            }
                            name = object.getString("name");
                            email = object.getString("email");
                            gender = object.getString("gender");
                            birthday = object.getString("birthday");
                        } catch (JSONException e) {
                            e.printStackTrace();
                        }
                    }
                    });

            saveSharedPreferences.loginPreferencesFb(email);
            Bundle parameters = new Bundle();
            parameters.putString("fields",
                    "id,name,email,gender,birthday");
            request.setParameters(parameters);
            request.executeAsync();
        }

        @Override
        public void onCancel() {
            // App code
        }

        @Override
        public void onError(FacebookException exception) {
            // App code
        }
    });
    startNewIntent();
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        callbackManager.onActivityResult(requestCode, resultCode, data);
    }
}

xml

<com.facebook.login.widget.LoginButton
    android:id="@+id/login_button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center_horizontal"
    android:layout_marginTop="30dp"
    android:layout_marginBottom="30dp" />