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);     } }