为什么用andriod studio添加进度条时，运行程序出现闪退？

Question

package com.example.mr18_xzvon.myapplication;

import android.app.ProgressDialog;
import android.content.Intent;
import android.net.Uri;
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.util.Log;
import android.view.KeyEvent;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.webkit.WebChromeClient;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    private String url="http://www.imooc.com/course/list";
    private WebView webView;
    private ProgressDialog dialog = new ProgressDialog(MainActivity.this);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.web);
        //Uri uri=Uri.parse(url);
        //Intent intent=new Intent(Intent.ACTION_VIEW,uri);
        //startActivity(intent);
        init();
    }

    private void init() {
        webView= (WebView) findViewById(R.id.webView);
        webView.loadUrl("http://www.imooc.com/course/list");
        //覆盖WebView默认通过第三方或者是浏览器打开网页的行为，使得网页可以在WebView中打开
        webView.setWebViewClient(new WebViewClient(){
            public boolean shouldOerridUrloading(WebView view,String url){
                //返回值是true的时候控制网页在WebView中去打开，如果为false调用系统浏览器或者第三方浏览器打开
                view.loadUrl(url);
                return true;
            }
            //WebViewClient帮助WebView去处理一些页面控制和请求通知
        });
        //启用支持Javascript
        WebSettings settings=webView.getSettings();
        settings.setJavaScriptEnabled(true);
        webView.setWebChromeClient(new WebChromeClient(){
            public  void noProgressChanged(WebView view,int newProgress){
                //newProgress 1-100之间的整数
                if(newProgress==100){
                    //网页加载完毕,关闭ProgressDialog
                   closeDialog();

                }else{
                    //网页正在加载，打开ProgressDialog
                   openDialog(newProgress);

                }
            }
            private void openDialog(int newProgress) {
                if (dialog==null){
                    dialog=new ProgressDialog(MainActivity.this);
                    dialog.setTitle("正在加载");
                    dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
                    dialog.setProgress(newProgress);
                    dialog.show();
                }
                else{
                    dialog.setProgress(newProgress);
                }
            }

            private void closeDialog() {
                if(dialog!=null&&dialog.isShowing()){
                    dialog.dismiss();
                    dialog=null;
                }

            }

        });
    }
    //改写物理按键--返回的逻辑
    public boolean onKeyDown(int keyCode,KeyEvent event){
        if(keyCode==KeyEvent.KEYCODE_BACK){
            if(webView.canGoBack()){
                Toast.makeText(this,webView.getUrl(),Toast.LENGTH_SHORT).show();
                webView.goBack();//返回上一页面
                return true;
            }
            else
            {
                System.exit(0);
            }
        }
        return super.onKeyDown(keyCode,event);
    }

}