Error when dropping an element from the Explorer on a diagram when there is no corresponding node description

[pcdavid]

Steps to reproduce

1. Create a model with a ConnectionUsage.
2. Open a diagram (GV or other).
3. Try to drop the ConnectionUsage on the diagram: 💥

Something went wrong while executing the tool 'Drop from Explorer'

Capture.video.du.2026-02-19.10-43-07.mp4

[pcdavid]

This is because DiagramMutationDndService.dropElementFromExplorerInTarget does not check whether the NodeDescription it searches for actually exists.

	java.util.NoSuchElementException: No value present
		at java.base/java.util.Optional.get(Optional.java:143)
		at org.eclipse.syson.diagram.services.DiagramMutationDndService.dropElementFromExplorerInTarget(DiagramMutationDndService.java:320)
		at org.eclipse.syson.diagram.services.DiagramMutationDndService.dropElementFromExplorer(DiagramMutationDndService.java:158)
		at org.eclipse.syson.diagram.services.aql.DiagramMutationAQLService.dropElementFromExplorer(DiagramMutationAQLService.java:159)