Add this to the additional mtouch arguments in the project's iOS Build options page:
--nosymbolstrip=ZBarReaderControllerResults
The difference between Debug and Release builds is that Release builds are stripped, thus removing the symbol for the field, so Xamarin.iOS can't find it at runtime. This option will make Xamarin.iOS tell the linker that it should keep that symbol, even if the symbol is not used (note that the binding to the field is a dynamic binding which happens at runtime, so the native strip
tool is not able to see that the field is actually used).