Dialog ID for welcome and finish pages is wrong into documentation of nsDialogs::Create function
http://nsis.sourceforge.net/Docs/nsDialogs/Readme.html#ref-create
It says "The Modern UI also has control 1040 for the welcome and the finish page." but the correct ID is 1044.
Thanks, fixed.
Documentation page still reports DialogID 1040...
http://nsis.sourceforge.net/Docs/nsDialogs/Readme.html#ref-create
It is fixed in the source tree, you are not going to see the fix on the live docs until we can get the nightly build going again...