This change adds tests for multiple related scenarios where the code generation was not including correct variable information, leading to these 'push null' problems. The new trackers for stack/variable usage when working with recent versions of Java are more careful in that it is not possible to manipulate stack/variable entries if they have not been previously described to the system. The Xwiki issue here uncovered two but there were a number of routes through the same method that needed testing and fixing (for example the code takes a different path for static vs non-static and private vs non-private). A snapshot build with these changes in seems to pass the Xwiki build process. Fixes #327