为何Accessibility Event中getRootInActiveWindow()始终返回null？

我已尝试了AccessibilityServiceConfig.xml的各类配置组合，测试设备为三星S21 Ultra，以下是相关代码及配置内容：

AccessibilityService 实现代码 #

@Override
public void onAccessibilityEvent(AccessibilityEvent event) {
    AccessibilityNodeInfo rootNode = getRootInActiveWindow();
}

@Override
protected void onServiceConnected(){
    super.onServiceConnected();
    AccessibilityServiceInfo info = new AccessibilityServiceInfo();
    info.eventTypes                 = AccessibilityEvent.TYPES_ALL_MASK;
    info.feedbackType               = AccessibilityServiceInfo.FEEDBACK_SPOKEN;
    info.notificationTimeout        = 1000;
    info.flags                      = AccessibilityServiceInfo.FLAG_REPORT_VIEW_IDS;
    this.setServiceInfo(info);
    Log.e(TAG,"onServiceConnected: ");
}

AccessibilityServiceConfig.xml 配置内容 #

<?xml version="1.0" encoding="utf-8"?>
<accessibility-service xmlns:android="http://schemas.android.com/apk/res/android"
    android:accessibilityEventTypes="typeWindowsChanged|typeWindowContentChanged|typeViewScrolled"
    android:accessibilityFlags="flagReportViewIds|flagDefault|flagRetrieveInteractiveWindows|flagIncludeNotImportantViews"
    android:canPerformGestures="true"
    android:canRetrieveWindowContent="true"
    android:canRequestFilterKeyEvents="false"
    android:accessibilityFeedbackType="feedbackAllMask"
    android:notificationTimeout="1000"
    android:canRequestEnhancedWebAccessibility="true">
</accessibility-service>

内容的提问来源于stack exchange，提问作者Yogesh Ghaturle