REL-2081: Spurious NPE in drag and drop operations.

Andy experienced spurious NPEs when removing observations from a plan. I was not able to reproduce this however it seemed that introducing an option for the result of `getDraggedObject()` might be a good idea anyway; there were already null checks in place for two of the three calls of this method, now empty results are checked for in all three usages. Manual testing seemed to work as expected, Andy will have to retest this with his data set.