Issue Title: Attempting to reorder UML AssociationEnd throws exception
Environment: MacOS 13.5. StarUML 5.1.0
Action Performed:
Step 1: Select an AssociationEnd in the Explorer using the context menu (i.e. right click) and select Move Up or Move Down
Expected Result: The AssociationEnd is reordered
Actual Result: No reordering and JS console reports error
/Applications/StarUM…ngine/engine.js:485 Uncaught TypeError: child.getOrdering is not a function
at /Applications/StarUM…ngine/engine.js:485
at /Applications/StarUM…dash/lodash.js:3782
at arrayMap (/Applications/StarUM…odash/lodash.js:653)
at /Applications/StarUM…dash/lodash.js:3781
at /Applications/StarUM…dash/lodash.js:3585
at /Applications/StarUM…dash/lodash.js:4967
at baseForOwn (/Applications/StarUM…dash/lodash.js:3032)
at /Applications/StarUM…dash/lodash.js:4936
at baseMap (/Applications/StarUM…dash/lodash.js:3584)
at baseOrderBy (/Applications/StarUM…dash/lodash.js:3780)