Cannot open drawer inside onReceivedError

[Bazley Bazzer]

I have this code within `onCreate`:

    webView.setWebViewClient(new WebViewClient() {

        final ProgressDialog progress = new ProgressDialog(MainActivity.this);
        .
        .
        public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
            view.loadUrl("file:///android_asset/www/error.html");
            DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
            drawer.openDrawer(GravityCompat.START);
            progress.dismiss();
        }

    });

When there is no internet connection it successfully detects the error and loads `error.html`. However, it refuses to open the drawer. Also, it closes the progress dialog whether I comment out that line or not. The drawer opens and closes fine when the back button is pressed:

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            drawer.openDrawer(GravityCompat.START);
        }
    }

Why will the drawer not open inside onReceivedError?