AddContentView to Unity's view somewhere turned black

I'm sorry for my poor English.After searching for some threads,it seems my situation is different.So i really need help ,i have no idea how to fix this.
I'm going to insert a WebView to the Activity with below lines:

Code (CSharp):

1.  
2.                 layout = new RelativeLayout(UnityPlayer.currentActivity.getApplicationContext());
3.                 RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(
4.                         RelativeLayout.LayoutParams.MATCH_PARENT,RelativeLayout.LayoutParams.MATCH_PARENT);
5.                 //MainActivity.this.addContentView(layout, params);
6.                 UnityPlayer.currentActivity.addContentView(layout, params);
7.                 WebView web = new WebView(UnityPlayer.currentActivity.getApplicationContext());
8.                 web.getSettings().setCacheMode(WebSettings.LOAD_NO_CACHE);
9.                 web.getSettings().setUseWideViewPort(false);
10.                 RelativeLayout.LayoutParams webparams = new RelativeLayout.LayoutParams(
11.                         width,height);
12.                 webparams.setMargins(leftMarginInt, bottomMarginInt, leftMarginInt, bottomMarginInt);
13.                 layout.addView(web,webparams);
14.          
15.                 web.loadUrl(uri);
16.                
17.                 layout.postInvalidate();
18.                 web.requestFocus();

However,"Sometimes" it looks like some where out of the WebView turned black , just after touch the screen everything goes well .Is Android kidding me?

 

OK，i'm still struggling .BUT , if i load the url which is sample enough(like google's home page),no more problem,everything goes well. so, if there is a way i can deal with the website will be one of the solutions.