[DMN Designer] Connector issues when no target node

Following merge of PRs for https://issues.redhat.com/browse/RHPAM-2319 a number of issues have been found with the DMN editor relating mainly to the use of a container node (Decision Service) and its children.

1) The attached video demonstrates a reproducible error although problems have also been encountered connecting child nodes to a contained node; but not in a readily reproducible way.

2) I was unable to create the attached diagram (showing a Decision Service with child nodes). I include a picture "before connectors" and "after connectors" (the "after connectors" is how it should be - I created that using a local build with commits for RHPAM-2319 rolled back. Creating this diagram was, at best, inconsistent using master... sometimes it seemd OK, othertimes it was impossible to connect (for example) "InputData-1" to "Decision-2" or "Decision-2" to "Decision-3" or "Decision-3" to "Decision-4".

3) I also observed the attached stack trace in the console during my attempts to create the diagram.