Reverted NULL assignments to obviate compilation warnings.

--
user: Michael Guravage <michael.guravage@cwi.nl>
branch 'default'

changed src/ChangeLog
changed src/VirtualLeaf.cpp
changed src/apoplastitem.cpp
changed src/build_models/ChangeLog
changed src/build_models/auxingrowthplugin.cpp
changed src/build_models/meinhardtplugin.cpp
changed src/build_models/testplugin.cpp
changed src/canvas.cpp
changed src/cell.cpp
changed src/cellbase.h
changed src/forwardeuler.cpp
changed src/mainbase.h
changed src/nodeitem.cpp
changed src/qcanvasarrow.h
changed src/simitembase.cpp