Child not allowed in org.zkoss.zkmax.zul.Cropper

Steps to Reproduce

1. run the code with 8.6.0.FL.20181019-Eval

    <cropper x="50" y="100" w="100" h="100" onCrop="img1.setContent(event.getImage())" toolbarVisible="true">
        <image src="swimming-pool.jpg"/>
    </cropper>
    <image id="img1"/>

Current Result

org.zkoss.zk.ui.UiException: Child not allowed in org.zkoss.zkmax.zul.Cropper
at org.zkoss.zk.ui.AbstractComponent.checkParentChild(AbstractComponent.java:1316)
at org.zkoss.zk.ui.AbstractComponent.insertBefore(AbstractComponent.java:1336)
at org.zkoss.zk.ui.impl.AbstractUiFactory.newComponent(AbstractUiFactory.java:140)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:919)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:889)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:776)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:740)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:934)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:889)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:776)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:836)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:794)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:740)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage0(UiEngineImpl.java:461)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage(UiEngineImpl.java:369)

Expected Result

no error

Debug Information

• the code is based on Component ref/cropper