Bug fix. It turned out the instance tree contained ghost instances, since InstanceTree.removeInstance sometimes failed. This caused those random crashes in UH. Now the InstanceTree enforces that remove/add Instance work in pairs. A new Exception is raised in case this ever goes wrong again. (InconsitencyDetected) Furthermore the removeInstancheChangeListener stuff had a fix to become reentrant. It is not clear wether this was shadowed by the aforementioned bug or was never triggered.