achartengine차트 draw그리기질문이요 제발 가르쳐주세요 ㅠㅠ

Question

public class ChartView extends View {
    private PieChart mPieChart = null;
    private MainActivity parent;
    public ChartView(Context context) {
        super(context);
        setFocusable(true);
        parent = (MainActivity) context;
       
    }
   
    public ChartView(Context context, AttributeSet attrs){
       
        super(context,attrs);
        setFocusable(true);
        parent = (MainActivity) context;
    }
   
    public ChartView(Context context, AttributeSet attrs, int defaultStyle)
    {
        super(context, attrs, defaultStyle);
        setFocusable(true);
        parent = (MainActivity) context;
    }
   
    public void makeChart(){
       
        double[] values = new double[] {10,20,30,40};
        int[] colors = new int[]{Color.CYAN, Color.MAGENTA,  Color.YELLOW, Color.GREEN};
        String[] texts = new String[] {"SAMPLE1", "SAMPEL2", "SAMPLE3", "SAMPLE4" };
      
       
        DefaultRenderer renderer = new DefaultRenderer();
        for(int color:colors){
            SimpleSeriesRenderer ssr = new SimpleSeriesRenderer();
            ssr.setColor(color);
            renderer.addSeriesRenderer(ssr);
        }
       
        //CategorySeries(java.lang.String title)
        CategorySeries series = new CategorySeries("계열 타이틀");
        int count = 0;
        for(double value: values){
            //add(java.lang.String category, double value)
            series.add(texts[count++],value);
        }
       
        mPieChart = new PieChart(series, renderer);
       
       
        invalidate();
    }
   
    @Override
    protected void onDraw(Canvas canvas)
    {
        int width = getMeasuredWidth();
        int height = getMeasuredHeight();
        Log.i("ChartView","onDraw->Width:"+width+"/height:"+height);
       
        if (mPieChart != null)
            mPieChart.draw(canvas, 0, 0, width-10, height-10,null);
      
    }
}
파이 차트 그리기를 공부중인데요 mPieChart.draw(canvas, 0, 0, width-10, height-10,null); 이부분에서 오류가 자꾸나와서 안되는데 mPieChart가 null값이 아니면 가득차게 그릴려고 하는건데 어떻게 해야 오류가 나지않을까요?? 고수님들 제발 가르쳐주세요 ㅠㅠ