AlertDialog - 无法添加窗口 - Android？

Question

我在webView中加载了一个网址，当禁用互联网连接显示对话框然后我按下后退按钮让我崩溃：

public class Guide extends AppCompatActivity {
    private WebView webview;
    private static final String TAG = "Main";
    private ProgressDialog progressBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        setTheme(R.style.AppThemeGreen);
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.guide);
        this.webview = (WebView) findViewById(R.id.webviewGuide);
        webview.getSettings().setJavaScriptEnabled(true);
        webview.getSettings().setRenderPriority(WebSettings.RenderPriority.HIGH);
        if (Build.VERSION.SDK_INT >= 19) {
            // chromium, enable hardware acceleration
            webview.setLayerType(View.LAYER_TYPE_HARDWARE, null);
        } else {
            // older android version, disable hardware acceleration
            webview.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
        }
        webview.getSettings().setCacheMode(WebSettings.LOAD_NO_CACHE);
        webview.setScrollBarStyle(WebView.SCROLLBARS_OUTSIDE_OVERLAY);

        final AlertDialog alertDialog = new AlertDialog.Builder(Guide.this).create();

        progressBar = ProgressDialog.show(Guide.this, getResources().getString(R.string.Waite), "Loading...");

        webview.setWebViewClient(new WebViewClient() {
            public boolean shouldOverrideUrlLoading(WebView view, String url) {
                Log.i(TAG, "Processing webview url click...");
                view.loadUrl(url);
                return true;
            }

            public void onPageFinished(WebView view, String url) {
                Log.i(TAG, "Finished loading URL: " + url);
                if (progressBar.isShowing()) {
                    progressBar.dismiss();
                }
            }

            public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                Log.e(TAG, "Error: " + description);
                Toast.makeText(Guide.this, "Oh no! " + description, Toast.LENGTH_SHORT).show();
                alertDialog.setTitle("Error");
                alertDialog.setMessage(description);
                alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        return;
                    }
                });
                alertDialog.show();
            }
        });
        webview.loadUrl(MYURL);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        if (webview != null) {
            webview.getSettings().setJavaScriptEnabled(false);
            webview.clearCache(true);
            webview.clearHistory();
        }
    }
}

这是我的日志：

04-22 21:57:05.174 10307-10854/net.tebyan.ir.sunofneinava E/cutils-trace: Error opening trace file: No such file or directory (2)
04-22 21:57:14.654 10307-10307/net.tebyan.ir.sunofneinava E/Main: Error: Couldn't find the URL.
04-22 21:57:14.714 10307-10307/net.tebyan.ir.sunofneinava E/AndroidRuntime: FATAL EXCEPTION: main
                                                                            android.view.WindowManager$BadTokenException: Unable to add window -- token android.os.BinderProxy@41cb00b8 is not valid; is your activity running?
                                                                                at android.view.ViewRootImpl.setView(ViewRootImpl.java:583)
                                                                                at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:269)
                                                                                at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:69)
                                                                                at android.app.Dialog.show(Dialog.java:281)
                                                                                at net.tebyan.ir.sunofneinava.Guide$1.onReceivedError(Guide.java:83)
                                                                                at android.webkit.CallbackProxy.handleMessage(CallbackProxy.java:353)
                                                                                at android.os.Handler.dispatchMessage(Handler.java:99)
                                                                                at android.os.Looper.loop(Looper.java:137)
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5148)
                                                                                at java.lang.reflect.Method.invokeNative(Native Method)
                                                                                at java.lang.reflect.Method.invoke(Method.java:525)
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
                                                                                at dalvik.system.NativeStart.main(Native Method)