I am trying to validate that popup window closes correctly
The window closes by:

@ContextParam(ContextType.VIEW)
private Component self;

self.detach();

Then in my ZATS test I close the window by:
desktop.query("button[label='Close']").click();

and then I query for this window and expect it to be null:
assertNull(desktop.query("#win"));

But instead I get this exception:
org.zkoss.zats.ZatsException
at org.zkoss.zats.mimic.impl.Searcher.findAll(Searcher.java:107)
at org.zkoss.zats.mimic.impl.Searcher.find(Searcher.java:118)
at org.zkoss.zats.mimic.impl.DefaultDesktopAgent.query(DefaultDesktopAgent.java:90)
<some calls in my code>
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:80)
at org.testng.internal.Invoker.invokeMethod(Invoker.java:702)
at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:894)
at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1219)
at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:127)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:111)
at org.testng.TestRunner.privateRun(TestRunner.java:768)
at org.testng.TestRunner.run(TestRunner.java:617)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:334)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:329)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:291)
at org.testng.SuiteRunner.run(SuiteRunner.java:240)
at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:53)
at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:87)
at org.testng.TestNG.runSuitesSequentially(TestNG.java:1185)
at org.testng.TestNG.runSuitesLocally(TestNG.java:1110)
at org.testng.TestNG.run(TestNG.java:1022)
at org.testng.remote.RemoteTestNG.run(RemoteTestNG.java:109)
at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:202)
at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:173)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.zkoss.zats.mimic.impl.Searcher.findAll(Searcher.java:74)
... 29 more
Caused by: java.lang.NullPointerException
at org.zkoss.zk.ui.select.impl.ComponentIterator.buildRootCtx(ComponentIterator.java:210)
at org.zkoss.zk.ui.select.impl.ComponentIterator.seekNext(ComponentIterator.java:192)
at org.zkoss.zk.ui.select.impl.ComponentIterator.loadNext(ComponentIterator.java:187)
at org.zkoss.zk.ui.select.impl.ComponentIterator.hasNext(ComponentIterator.java:146)
at org.zkoss.zk.ui.select.Selectors.toList(Selectors.java:341)
at org.zkoss.zk.ui.select.Selectors.find(Selectors.java:86)
... 34 more

This happens because the code in ComponentIterator does not expect that there may be no root components at all e.g. that that desktop is empty:

package org.zkoss.zk.ui.select.impl;

public class ComponentIterator implements Iterator<Component> {

private ComponentMatchCtx buildRootCtx() {
Component rt = _root == null ? _page.getFirstRoot() : _root;

Component rt2 = rt.getFellowIfAny(seq.getId());

Please have a look and fix