flutter_webview_plugin webview 释放
/android/src/main/java.com.flutter_webview_plugin/WebviewManager
line 462:

void close(MethodCall call, MethodChannel.Result result) {
        if (webView != null) {
            ViewGroup vg = (ViewGroup) (webView.getParent());
            vg.removeView(webView);
            // 进行销毁
            webView.removeAllViews();
            webView.destroy();
        }
        webView = null;
        if (result != null) {
            result.success(null);
        }

        closed = true;
        FlutterWebviewPlugin.channel.invokeMethod("onDestroy", null);
    }