Currently a component detached during dragging (server push or rod or timer) causes a ComponentNotFound exception when creating the onDrop event.

The FR is to provide an optional switch (e.g. library property) to allow a null value for the dragged component.

https://github.com/zkoss/zk/blob/v8.6.1/zk/src/org/zkoss/zk/ui/event/DropEvent.java#L43

Using getComponentByUuidIfAny will allow for null values, in case the option is enabled.