The reproductions steps are simple:
Have a node with 3 children (e.g. create 3 empty segmentations), drag the top two children and drop them on the third -> crash.
Fix seems incredibly simple:
Change line 198 here: https://github.com/MITK/MITK/blob/master/Modules/QtWidgets/src/QmitkDataStorageTreeModel.cpp#L198
from:
if (dropIndex == -1)
to:
if (dropIndex == -1 || dropIndex > parentItem->GetChildCount())