为什么把ObjectAnimator设置成成员变量，调用start() 报错呢 400 请求报错 

package com.example.azezer0.demoanimatior;

import android.animation.ObjectAnimator;
import android.animation.PropertyValuesHolder;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
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.ImageView;

public class MainActivity extends AppCompatActivity {
    private ImageView iv;

    private ObjectAnimator checkAnimator;
    private PropertyValuesHolder propertyValueScaleX;
    private PropertyValuesHolder propertyValueScaleY;
    private PropertyValuesHolder propertyValueScaleAlpha;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iv = (ImageView) findViewById(R.id.iv);
        initAnimator();
        iv.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                checkAnimator.start();
            }
        });

    }

    private void initAnimator() {
        checkAnimator = new ObjectAnimator();
        propertyValueScaleX = PropertyValuesHolder.ofFloat("scaleX", 0.0f, 1.0f);
        propertyValueScaleY = PropertyValuesHolder.ofFloat("scaleY", 0.0f, 1.0f);
        propertyValueScaleAlpha = PropertyValuesHolder.ofFloat("alpha", 0.0f, 1.0f);
        checkAnimator.ofPropertyValuesHolder(iv, propertyValueScaleAlpha, propertyValueScaleX, propertyValueScaleY).setDuration(2000).start();
    }


}