Steps to Reproduce

Run fiddle
http://zkfiddle.org/sample/ac8p6b/1-include-defer-mode-fails-changed-while-loading

• on page load, the include receive src="in.zul?val=1"
• during the rendering of in.zul, the include src is updated to src="in.zul?val=2"
  (include is in auto mode, which act as defer due to ?params in the url)

Current Result

include returns

Cause: Unknown exception: java.lang.NullPointerException.
java.lang.NullPointerException
	at org.zkoss.zk.ui.sys.HtmlPageRenders.outPageContent(HtmlPageRenders.java:583)

Expected Result

include should output the 2nd page

Debug Info

Sequence of events:

• Outer page is loaded.
• Include in defer mode is instantiated
• Include does
  https://github.com/zkoss/zk/blob/master/zk/src/org/zkoss/zk/ui/http/ExecutionImpl.java#L201
  org.zkoss.zk.ui.http.ExecutionImpl.include(Writer, String, Map<String, ?>, int)
  to output the included page (innerpage#1)
• While the page is rendering, something happen in the inner page which changes the source of the outer include component. (in fiddle, it's a direct setter, but can be any indirect action such as event queues, commands, global commands, callbacks, etc...)
• include.setSrc will trigger invalidate on include, and create a new inner page (innerpage#2)
  https://github.com/zkoss/zk/blob/master/zul/src/org/zkoss/zul/Include.java#L295
• Invalidated include will cause pageImpl.destroy() to trigger on innerpage#1, which removes the desktop from innerpage#1
• innerpage#2 will finish rendering and return.
• innerpage#1 will continue rendering and fail, since it now missing a desktop, causing NPE and preventing the innerpage#2 from being rendered.
  --Note: in this scenario, even if the desktop exception didn't happen, the resulting page would be innerpage#1, since it will finish resolving after innerpage#2 and overwrites the result.

Root Cause

Include in defer mode source is changed while the first include is still rendering the page

Workaround