restoreToCount 异常报错.

 int save = canvas.saveLayer(null, null, LAYER_FLAG);


        int iWidth = mProgressDrawable.getMinimumWidth();
        int iHeight = mProgressDrawable.getMinimumHeight();
        if (iWidth == 0) iWidth = MIN_SIZE;
        if (iHeight == 0) iHeight = MIN_SIZE;

        mDrawRect.right = iWidth;
        mDrawRect.bottom = iHeight;
        RectF rect = mDrawRect;
        mPaint.setColor(0xFFFFFFFF);
        canvas.drawArc(rect, mStartAngle, mSweepAngle, true, mPaint);
        mPaint.setXfermode(mDuffXfermode);

        if(mMemeryBmp==null)
        {
            mMemeryBmp = Bitmap.createBitmap(iWidth,iHeight,Bitmap.Config.ARGB_8888);
            mMemeryCanvas = new Canvas(mMemeryBmp);
        }
        mMemeryCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
        mProgressDrawable.setBounds(0, 0, iWidth, iHeight);
        mProgressDrawable.draw(mMemeryCanvas);
        canvas.drawBitmap(mMemeryBmp,null,rect, mPaint);

        mPaint.setXfermode(null);
        canvas.restoreToCount(save);

canvas.restoreToCount(save); 这一句异常报错.

栈如下

java.lang.NullPointerException
at android.graphics.Canvas_Delegate.restoreTo(Canvas_Delegate.java:1069)
at android.graphics.Canvas_Delegate.native_restoreToCount(Canvas_Delegate.java:249)
at android.graphics.Canvas.native_restoreToCount(Canvas.java)
at android.graphics.Canvas.restoreToCount(Canvas.java:566)
at android.view.View.draw(View.java:16115)
at android.view.ViewGroup.drawChild_Original(ViewGroup.java:3609)
at android.view.ViewGroup_Delegate.drawChild(ViewGroup_Delegate.java:61)
at android.view.ViewGroup.drawChild(ViewGroup.java:3609)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399)
at android.view.View.draw(View.java:16088)
at android.view.ViewGroup.drawChild_Original(ViewGroup.java:3609)
at android.view.ViewGroup_Delegate.drawChild(ViewGroup_Delegate.java:61)
at android.view.ViewGroup.drawChild(ViewGroup.java:3609)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399)
at android.view.View.draw(View.java:16181)
at android.view.View.draw(View.java:16090)
at android.view.ViewGroup.drawChild_Original(ViewGroup.java:3609)
at android.view.ViewGroup_Delegate.drawChild(ViewGroup_Delegate.java:61)
at android.view.ViewGroup.drawChild(ViewGroup.java:3609)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399)
at android.view.View.draw(View.java:16088)
at android.view.ViewGroup.drawChild_Original(ViewGroup.java:3609)
at android.view.ViewGroup_Delegate.drawChild(ViewGroup_Delegate.java:61)
at android.view.ViewGroup.drawChild(ViewGroup.java:3609)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3399)
at android.view.View.draw(View.java:16181)
at com.android.layoutlib.bridge.impl.RenderSessionImpl.render(RenderSessionImpl.java:455)
at com.android.layoutlib.bridge.Bridge.createSession(Bridge.java:428)
at com.android.ide.common.rendering.LayoutLibrary.createSession(LayoutLibrary.java:350)
at com.android.tools.idea.rendering.RenderTask$2.compute(RenderTask.java:520)
at com.android.tools.idea.rendering.RenderTask$2.compute(RenderTask.java:508)
at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:967)
at com.android.tools.idea.rendering.RenderTask.createRenderSession(RenderTask.java:508)
at com.android.tools.idea.rendering.RenderTask.access$600(RenderTask.java:75)
at com.android.tools.idea.rendering.RenderTask$3.call(RenderTask.java:620)
at com.android.tools.idea.rendering.RenderTask$3.call(RenderTask.java:617)
at com.android.tools.idea.rendering.RenderService.runRenderAction(RenderService.java:371)
at com.android.tools.idea.rendering.RenderTask.render(RenderTask.java:617)
at com.android.tools.idea.rendering.RenderTask.render(RenderTask.java:639)
at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager.doRender(AndroidLayoutPreviewToolWindowManager.java:654)
at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager.access$1700(AndroidLayoutPreviewToolWindowManager.java:82)
at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager$7$1.run(AndroidLayoutPreviewToolWindowManager.java:596)
at com.intellij.openapi.progress.impl.CoreProgressManager$2.run(CoreProgressManager.java:142)
at com.intellij.openapi.progress.impl.CoreProgressManager.registerIndicatorAndRun(CoreProgressManager.java:446)
at com.intellij.openapi.progress.impl.CoreProgressManager.executeProcessUnderProgress(CoreProgressManager.java:392)
at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:54)
at com.intellij.openapi.progress.impl.CoreProgressManager.runProcess(CoreProgressManager.java:127)
at org.jetbrains.android.uipreview.AndroidLayoutPreviewToolWindowManager$7.run(AndroidLayoutPreviewToolWindowManager.java:591)
at com.intellij.util.ui.update.MergingUpdateQueue.execute(MergingUpdateQueue.java:337)
at com.intellij.util.ui.update.MergingUpdateQueue.execute(MergingUpdateQueue.java:327)
at com.intellij.util.ui.update.MergingUpdateQueue$3.run(MergingUpdateQueue.java:271)
at com.intellij.util.ui.update.MergingUpdateQueue.flush(MergingUpdateQueue.java:286)
at com.intellij.util.ui.update.MergingUpdateQueue.flush(MergingUpdateQueue.java:244)
at com.intellij.util.ui.update.MergingUpdateQueue.run(MergingUpdateQueue.java:234)
at com.intellij.util.concurrency.QueueProcessor.runSafely(QueueProcessor.java:238)
at com.intellij.util.Alarm$Request$1.run(Alarm.java:352)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)