HarmonyOS 使用AbilityDelegatorRegistry的getCurrentTopAbility方法，获取不到当前应用顶部ability

需要拿到当前应用顶部ability，查阅文档后发现有AbilityDelegatorRegistry的getCurrentTopAbility方法，拿到的abilityDelegator是null。

参考文档：

https://developer.huawei.com/consumer/cn/doc/harmonyos-references-V5/js-apis-inner-application-abilitydelegator-V5#getcurrenttopability9

代码如下：

import KsIntegralTimeTaskUtils from '../tools/KsIntegralTimeTaskUtils';
import KsEventsManager from './KsEventsManager';
import { BusinessError } from '@kit.BasicServicesKit';

/**
 * 获取当前页面实例
 */
public async getCurrentTopAbility(): Promise<UIAbility | undefined> {
  let temp: Promise<UIAbility | undefined> = new Promise((resolve: Function) => {
    let abilityDelegator: AbilityDelegatorRegistry.AbilityDelegator = AbilityDelegatorRegistry.getAbilityDelegator();
    if (abilityDelegator) {
      abilityDelegator.getCurrentTopAbility((err: BusinessError, data: UIAbility) => {
        resolve(data)
      });
    } else {
    resolve(undefined)
    }
  })
  return temp;
}

在EntryAbility里调用定义的getCurrentTopAbility方法。