使用HTML5WebView时出现空指针异常

Question

我正在使用HTML5WebView播放vimeo视频。首先，我在列表视图中显示视频列表，然后单击其中一个项目，打开使用HTML5WebView播放视频的活动。它适用于第一次播放，并为任何其他vimeo视频提供例外。例外情况如下：

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11-17 10:35:14.989: E/AndroidRuntime(30576): FATAL EXCEPTION: main
11-17 10:35:14.989: E/AndroidRuntime(30576): java.lang.NullPointerException
11-17 10:35:14.989: E/AndroidRuntime(30576):    at android.webkit.HTML5VideoView.setVolume(HTML5VideoView.java:219)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at android.webkit.HTML5VideoViewProxy$VideoPlayer.setVolume(HTML5VideoViewProxy.java:326)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at android.webkit.HTML5VideoViewProxy.handleMessage(HTML5VideoViewProxy.java:488)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at android.os.Handler.dispatchMessage(Handler.java:99)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at android.os.Looper.loop(Looper.java:137)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at android.app.ActivityThread.main(ActivityThread.java:4950)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at java.lang.reflect.Method.invokeNative(Native Method)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at java.lang.reflect.Method.invoke(Method.java:511)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
11-17 10:35:14.989: E/AndroidRuntime(30576):    at dalvik.system.NativeStart.main(Native Method)

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HTML5WebView代码（我从其中一个github repos获得）如下：

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public class Html5WebView extends WebView {

private Context mContext;
private MyWebChromeClient mWebChromeClient;
private View mCustomView;
private FrameLayout mCustomViewContainer;
private WebChromeClient.CustomViewCallback mCustomViewCallback;

private FrameLayout mContentView;
private FrameLayout mBrowserFrameLayout;
private FrameLayout mLayout;

static final String LOGTAG = "HTML5WebView";

public Html5WebView(Context context) {
    super(context);
    init(context);
}


public Html5WebView(Context context, AttributeSet attrs) {
    super(context, attrs);
    init(context);
}

public Html5WebView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    init(context);
}

private void init(Context context) {
    mContext = context;
    Activity a = (Activity) mContext;

    mLayout = new FrameLayout(context);

    mBrowserFrameLayout = (FrameLayout) LayoutInflater.from(a).inflate(
            R.layout.custom_screen, null);
    mContentView = (FrameLayout) mBrowserFrameLayout
            .findViewById(R.id.main_content);
    mCustomViewContainer = (FrameLayout) mBrowserFrameLayout
            .findViewById(R.id.fullscreen_custom_content);

    mLayout.addView(mBrowserFrameLayout, COVER_SCREEN_PARAMS);

    // Configure the webview
    WebSettings s = getSettings();
    s.setBuiltInZoomControls(true);
    s.setLayoutAlgorithm(WebSettings.LayoutAlgorithm.NARROW_COLUMNS);
    s.setUseWideViewPort(true);
    s.setLoadWithOverviewMode(true);
    // s.setSavePassword(true);
    s.setSaveFormData(true);
    s.setJavaScriptEnabled(true);
    mWebChromeClient = new MyWebChromeClient();
    setWebChromeClient(mWebChromeClient);

    setWebViewClient(new WebViewClient());

    setScrollBarStyle(View.SCROLLBARS_INSIDE_OVERLAY);

    // enable navigator.geolocation
    // s.setGeolocationEnabled(true);
    // s.setGeolocationDatabasePath("/data/data/org.itri.html5webview/databases/");

    // enable Web Storage: localStorage, sessionStorage
    s.setDomStorageEnabled(false);

    mContentView.addView(this);
}

public FrameLayout getLayout() {
    return mLayout;
}

public boolean inCustomView() {
    return (mCustomView != null);
}

public void hideCustomView() {
    mWebChromeClient.onHideCustomView();
}

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        if ((mCustomView == null) && canGoBack()) {
            goBack();
            return true;
        }
    }
    return super.onKeyDown(keyCode, event);
}

private class MyWebChromeClient extends WebChromeClient {
    private Bitmap mDefaultVideoPoster;
    private View mVideoProgressView;

    @Override
    public void onShowCustomView(View view,
            WebChromeClient.CustomViewCallback callback) {
        // Log.i(LOGTAG, "here in on ShowCustomView");
        Html5WebView.this.setVisibility(View.GONE);

        // if a view already exists then immediately terminate the new one
        if (mCustomView != null) {
            callback.onCustomViewHidden();
            return;
        }

        mCustomViewContainer.addView(view);
        mCustomView = view;
        mCustomViewCallback = callback;
        mCustomViewContainer.setVisibility(View.VISIBLE);
    }

    @Override
    public void onHideCustomView() {
        System.out.println("customview hideeeeeeeeeeeeeeeeeeeeeeeeeee");
        if (mCustomView == null)
            return;

        // Hide the custom view.
        mCustomView.setVisibility(View.GONE);

        // Remove the custom view from its container.
        mCustomViewContainer.removeView(mCustomView);
        mCustomView = null;
        mCustomViewContainer.setVisibility(View.GONE);
        mCustomViewCallback.onCustomViewHidden();

        Html5WebView.this.setVisibility(View.VISIBLE);
        Html5WebView.this.goBack();
        // Log.i(LOGTAG, "set it to webVew");
    }

    @Override
    public View getVideoLoadingProgressView() {
        // Log.i(LOGTAG, "here in on getVideoLoadingPregressView");

        if (mVideoProgressView == null) {
            LayoutInflater inflater = LayoutInflater.from(mContext);
            mVideoProgressView = inflater.inflate(
                    R.layout.video_loading_progress, null);
        }
        return mVideoProgressView;
    }

    @Override
    public void onReceivedTitle(WebView view, String title) {
        ((Activity) mContext).setTitle(title);
    }

    @Override
    public void onProgressChanged(WebView view, int newProgress) {
        ((Activity) mContext).getWindow().setFeatureInt(
                Window.FEATURE_PROGRESS, newProgress * 100);
    }

    @Override
    public void onGeolocationPermissionsShowPrompt(String origin,
            GeolocationPermissions.Callback callback) {
        callback.invoke(origin, true, false);
    }
}

static final FrameLayout.LayoutParams COVER_SCREEN_PARAMS = new FrameLayout.LayoutParams(
        ViewGroup.LayoutParams.MATCH_PARENT,
        ViewGroup.LayoutParams.MATCH_PARENT);
}

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在活动中，我只是拥有这么多代码： （该ID是来自带有视频列表的活动的vimeo视频ID。）

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 public static class PlaceholderFragment extends Fragment {

    private Html5WebView mWebView;
    private String url;
    public PlaceholderFragment() {

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        mWebView = new Html5WebView(getActivity());
        url = "http://player.vimeo.com/video/"+id;
        System.out.println("url::"+url);
        mWebView.loadUrl(url);
        return mWebView.getLayout();
    }
}