HarmonyOS webPageSnapshot api调用失败

可能是作用域的问题，参考以下示例

// xxx.ets
import { webview } from '@kit.ArkWeb';
import { BusinessError } from '@kit.BasicServicesKit';
import common from '@ohos.app.ability.common';

@Entry
@Component
struct PageSnapshotPage {
  controller: webview.WebviewController = new webview.WebviewController();

  init () {
    let uiAbilityContext = this.getUIContext().getHostContext()    as common.UIAbilityContext
    let windowStage = uiAbilityContext.windowStage
    let webViewCtrl = this.controller
    let mainWindow = windowStage.getMainWindowSync();
    if(mainWindow) {
      mainWindow.on('screenshot', () => {
        try {
          webViewCtrl.webPageSnapshot({ id: "1234", size: { width: 100, height: 100 } }, (error, result) => {
            if (error) {
              console.error(`ErrorCode: ${(error as BusinessError).code},  Message: ${(error as BusinessError).message}`);
              return;
            }
            if (result) {
              console.info(`