ImageDataItems represent different image parts (e.g. slice, volume, channel) and point to corresponding memory areas. Unfortunately, ImageDataItems aren’t available at all times, they are created on demand. This behavior impedes several aspects from const-correctness to thread-safety and therefore blocks several image bugs.
I recommend removing this on-demand behavior in order to make progress regarding mitk::Image and its legacies. The Initialization of images would then require an initialization of all necessary/possible ImageDataItems.