I found out that this is a basic problem of changing of parent nodes. It can also be reproduced in a much more simpler way:

1. Open Pic3D
2. Open US4DCyl
3. Make one image the child node of the other image
4. Crash

This did not happen in MITK v2018.04.2

Sorry for blaming here but I had some suspicions and indeed, the bug was introduced here: 8dea40a4a757

Currently I can't see if this really introduced the bug or made an already existing bug visible.

My first guess is:
in line 305 of QmitkDataStorageTreeModel::dropMimeData
mitk::DataNode *droppedNode = (*diIter)->GetDataNode();
the dropped node is returned, which is invalidated after line 307:
dataStorage->Remove(droppedNode);
which leads to an error in line 308:
dataStorage->Add(droppedNode, dropOntoNode);
because droppedNode does not exist anymore.

This was not a problem before, because the raw pointer still existed but now the weak pointer frees the memory? Something like that.
One solution for this could be to replace the raw pointer in line 305 with a smart pointer
mitk::DataNode::Pointer droppedNode = (*diIter)->GetDataNode();

But maybe I'm wrong and something is not fine with QmitkDataStorageTreeModelInternalItem::GetDataNode().

Btw.: QmitkDataStorageTreeModel::dropMimeData is waaay to long and complicated - no wonder there might be some bugs hidden.

I agree that the correct solution is to use a smart pointer in line 305 instead of a raw pointer. Ralf did not introduce a bug but made another one visible. :-)

I looked into the code and aggree on both:

1. Using a smartpointer at L305 makes sense, not using it is hazardous.
2. Ralf does not introduce errors 😄

In T27559#206800, @floca wrote:

Ralf does not introduce errors 😄

This reads like a general statement 😛

In T27559#206803, @kalali wrote:

In T27559#206800, @floca wrote:

Ralf does not introduce errors 😄

This reads like a general statement 😛

Start at 16 seconds, end at 30 seconds (sorry @floca):

In T27559#206852, @kislinsk wrote:

Start at 16 seconds, end at 30 seconds (sorry @floca):

ROFL (ralf on the floor laughing)
😆