Android crash when use WebView to watch video in separate view

Question

I have a fragment using a RelativeLayout to hold a webview, and assigned a custom WebChromeClient to it. When I use the webview to watch Youtube video, it can shows video in a separate view. However, when I exit from the separate view and go back to webview, the app crashes.

/*Here is part of my WebChromeClient*/
public class CustomWebChromeClient extends WebChromeClient {

    /*Function for showing video separate from webview*/
    public void onShowCustomView(View view, CustomViewCallback callback) {
        frag.mCustomViewCallback = callback;

        //mTargetView is used as a video holder for holding the video object
        frag.mTargetView.addView(view);
        //mCustomView is the video object itself
        frag.mCustomView = view;
        //hide webview      
        frag.webview.setVisibility(View.GONE);  
        //show video holder with its video
        frag.mTargetView.setVisibility(View.VISIBLE);

        frag.mTargetView.bringToFront();

    }

    @Override
    public void onHideCustomView() {

        if (frag.mCustomView == null)
            return;

        //(I guess) use callback to hide video player widget such as play button and timebar
        frag.mCustomViewCallback.onCustomViewHidden();
        //remove video object from holder
        frag.mTargetView.removeView(frag.mCustomView);
        //hide video
        frag.mCustomView.setVisibility(View.GONE);
        frag.mCustomView = null;
        //hide video hodler
        frag.mTargetView.setVisibility(View.GONE);
        //show webview again
        frag.webview.setVisibility(View.VISIBLE);


    }

    /*... some other codes ...*/
}   

Inside my main fragment activity

/*Hide video holder when back key pressed*/
public void onBackPressed() {
    /*if is showing video, hide the view*/
    if(isShowingVideo){
        myWebChromeClient.onHideCustomView();
    }
}

Detail situation:
1. in my webview
2. Open Youtube
3. Go to one of the video page
4. Click on video's play icon
5. [Important]While it is still loading(remain black on the video),click fullscreen button it provided.
6. triggered onShowCustomView function, and then a fullscreen video is showing(hide webview and show video view only)
7. some seconds of video play
8. press back key, and triggered onHideCustomView.(hide video view and show webview only)
9. webview's video has resize and about 1~2 seconds, the app crashed and get some error messages.

Error message:
Report WebCore crash to the ErrorReportUtils at:

Thu Dec 11 17:37:11
Fatal signal 11 (SIGSEGV) at 0x000030de (code=0), thread 12526 (WebViewCoreThre)
[unnamed-12510-1] updateTexImage: SurfaceTexture is not attached to an OpenGL ES context

What have I discovered:
1. without using "mCustomViewCallback.onCustomViewHidden();" inside onHideCustomView(), app won't crash,
but video player widget created by mCustomView(the video object) still exists.
2. In situation 5, if I wait until video content shows, then click fullscreen button, app works perfectly after situation 6~8.