org.zkoss.zul.drop.allowNullDragged
From Documentation
Property:
org.zkoss.zul.drop.allowNullDragged
Applicable: globally in zk.xml via <library-property> not as <custom-attribute>
Default: false
Since 8.6.2
If the value is true, then the DropEvent will allow NULL Component without throwing an exception, which means the dragged component was detached or destroyed.