When calling SinglePointDataInteractor::SetDataNode(node) with a data node in which already a point is stored, that point is cleared which, in my opinion, is incorrect behavior.
The error occurred on MITK version 2018-04 but looking at the latest development version of SinglePointDataInteractor the faulty line of code is still in there.
Proposal for correction:
In mitkSinglePointDataInteractor.cpp:
remove the line points->Clear(); in the else clause (line 103 in 2018-4 version).