Pasar variable a funcion de otra clase

[sototbb]

Buenos dias, les cuento que estoy aprendiendo android y copiando uno de los ejemplos que es un "Paint" me doy con un error de NullPointerException.

Codigo Main Activity

Código:

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ImageButton;


public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    ImageButton negro,blanco,rojo,verde,azul;
    public static Lienzo lienzo;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        negro = (ImageButton) findViewById(R.id.Negro);
        blanco = (ImageButton) findViewById(R.id.Blanco);
        rojo = (ImageButton) findViewById(R.id.Rojo);
        verde = (ImageButton) findViewById(R.id.Verde);
        azul = (ImageButton) findViewById(R.id.Azul);

        negro.setOnClickListener(this);
        blanco.setOnClickListener(this);
        rojo.setOnClickListener(this);
        verde.setOnClickListener(this);
        azul.setOnClickListener(this);

        lienzo = (Lienzo) findViewById(R.id.lienzo);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onClick(View view) {
    String color=null;
        switch (view.getId()){
            case (R.id.Negro):
                color= view.getTag().toString();
                lienzo.setColor(color);
                break;
            case (R.id.Blanco):
                color= view.getTag().toString();
                lienzo.setColor(color);
                break;
            case (R.id.Rojo):
                color= view.getTag().toString();
                lienzo.setColor(color);
                break;
            case (R.id.Verde):
                color= view.getTag().toString();
                lienzo.setColor(color);
                break;
            case (R.id.Azul):
                color= view.getTag().toString();
                lienzo.setColor(color);
                break;
            default:
                break;
        }
    }

}

Codigo Lienzo:

Código:

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class Lienzo extends View{

    //Path que utilizara para pintar las lineas
    private Path drawPath;
    //Paint de dibujar y paint de canvas
    private Paint drawPaint,canvasPaint;
    //Color inicial
    private int paintColor = 0xFFFF0000;
    //Canvas
    private Canvas drawCanvas;
    //Canvas para guardar
    private Bitmap canvasBitmap;

    public Lienzo(Context context, AttributeSet attrs) {
        super(context, attrs);
        setupDrawing();
    }


    private void setupDrawing(){
        //Configuracion del area sobre la que va a pintar
        drawPath = new Path();
        drawPaint = new Paint();
        drawPaint.setColor(paintColor);
        drawPaint.setAntiAlias(true);
        drawPaint.setStrokeWidth(20);
        drawPaint.setStyle(Paint.Style.STROKE);
        drawPaint.setStrokeJoin(Paint.Join.ROUND);
        drawPaint.setStrokeCap(Paint.Cap.ROUND);
        canvasPaint = new Paint(Paint.DITHER_FLAG);
    }



    //Tamaño asignado a la vista cuando crea el objeto
    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh){
        super.onSizeChanged(w,h,oldw,oldh);
        canvasBitmap = Bitmap.createBitmap(w,h,Bitmap.Config.ARGB_8888);
        drawCanvas = new Canvas(canvasBitmap);
    }


    //Pinta la vista. Sera llamado desde el onTouch

    @Override
    protected void onDraw(Canvas canvas){
        canvas.drawBitmap(canvasBitmap,0,0,canvasPaint);
        canvas.drawPath(drawPath,drawPaint);
    }

    //Registra los Touch del usuario
    @Override
    public boolean onTouchEvent(MotionEvent event){
        float touchX = event.getX();
        float touchY = event.getY();

        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                drawPath.moveTo(touchX,touchY);
                break;
            case MotionEvent.ACTION_MOVE:
                drawPath.lineTo(touchX,touchY);
                break;
            case MotionEvent.ACTION_UP:
                drawPath.lineTo(touchX,touchY);
                drawCanvas.drawPath(drawPath,drawPaint);
                drawPath.reset();
                break;
            default:
                return false;
        }

        //Repintar
        invalidate();
        return true;
    }

    //Actualiza color
    public void setColor (String newColor){
        invalidate();
        paintColor = Color.parseColor(newColor);
        drawPaint.setColor(paintColor);
    }

}

Codigo Layout:

Código:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.example.nicolas.paint.MainActivity"
    tools:showIn="@layout/activity_main">

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="TextView" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="2"
        android:orientation="horizontal">

        <ImageButton
            android:id="@+id/button"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:src="@drawable/ic_stat_name" />

        <ImageButton
            android:id="@+id/button2"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:src="@drawable/trazo" />

        <ImageButton
            android:id="@+id/button3"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:src="@drawable/borrar" />

        <ImageButton
            android:id="@+id/button4"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:src="@drawable/guardar" />

    </LinearLayout>

    <view
        class="com.example.nicolas.paint.Lienzo"
        id="@+id/lienzo"
        layout_height="0dp"
        layout_weight="1"
        layout_width="match_parent"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="12"
        android:background="#FFFFFFFF" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal">

        <ImageButton
            android:id="@+id/Negro"
            android:layout_width="0dp"
            android:layout_height="@dimen/alturacolor"
            android:layout_weight="1"
            android:background="#FF000000"
            android:tag="#FF000000" />

        <ImageButton
            android:id="@+id/Blanco"
            android:layout_width="0dp"
            android:layout_height="@dimen/alturacolor"
            android:layout_weight="1"
            android:background="#FFFFFFFF"
            android:tag="#FFFFFFFF" />

        <ImageButton
            android:id="@+id/Azul"
            android:layout_width="0dp"
            android:layout_height="@dimen/alturacolor"
            android:layout_weight="1"
            android:background="#FF0000FF"
            android:tag="#FF0000FF" />

        <ImageButton
            android:id="@+id/Rojo"
            android:layout_width="0dp"
            android:layout_height="@dimen/alturacolor"
            android:layout_weight="1"
            android:background="#FFFF0000"
            android:tag="#FFFF0000" />

        <ImageButton
            android:id="@+id/Verde"
            android:layout_width="0dp"
            android:layout_height="@dimen/alturacolor"
            android:layout_weight="1"
            android:background="#FF00FF00"
            android:tag="#FF00FF00" />
    </LinearLayout>

</LinearLayout>

Desde ya gracias