As usual while waiting for the next release - don't forget to check the nightly builds in the forum.
<wxPanel wxs="wxsmith/wxsTreebookParentQP.wxs" src="wxwidgets/defitems/wxstreebook.cpp" hdr="wxwidgets/defitems/wxstreebook.cpp" name="wxsTreebookParentQP" language="CPP" />
It seems never existed?