NPE during hierarchical layout with fixed order ports

[uruuru]

The graph below triggers the exception below.

Removing fixed order port constraints from n2_1 allows a proper layout.

graph root
hierarchyHandling: INCLUDE_CHILDREN
algorithm: layered
node n1 {
	layout [ size: 50, 50 ]
	elk.padding: "[top=50.0,left=50.0,bottom=50.0,right=50.0]"
	portConstraints: FIXED_ORDER
	label "n1" {
		layout [ size: 8, -9.5 ]
	}
} 
node n2 {
	layout [ size: 190, 150 ]
	elk.padding: "[top=120.0,left=50.0,bottom=50.0,right=50.0]"
	portConstraints: FIXED_ORDER
	node n2_1 {
		layout [ size: 80, 80 ]
		elk.padding: "[top=50.0,left=50.0,bottom=50.0,right=50.0]"
		portConstraints: FIXED_ORDER
		node n2_1_1 {
			layout [ size: 80, 80 ]
			elk.padding: "[top=50.0,left=50.0,bottom=50.0,right=50.0]"
			portConstraints: FIXED_ORDER
		}
	}
	node n2_2 {
		layout [ size: 80, 80 ]
		elk.padding: "[top=50.0,left=50.0,bottom=50.0,right=50.0]"
		portConstraints: FIXED_ORDER
	}
	edge e2_1_2_2: n2_1 -> n2_2
}
node n3 {
	layout [ size: 50, 50 ]
	elk.padding: "[top=50.0,left=50.0,bottom=50.0,right=50.0]"
	portConstraints: FIXED_ORDER
	label "n3" {
		layout [ size: 8, -9.5 ]
	}
}
edge e12_1: n1 -> n2.n2_1
edge e1_2_1_1: n1 -> n2.n2_1.n2_1_1
edge e2_2_3: n2.n2_2 -> n3

java.lang.NullPointerException
   at org.eclipse.elk.alg.layered.graph.LGraphUtil.createExternalPortDummy(LGraphUtil.java:937)
   at org.eclipse.elk.alg.layered.compound.CompoundGraphPreprocessor.transformHierarchyEdges(CompoundGraphPreprocessor.java:176)
   at org.eclipse.elk.alg.layered.compound.CompoundGraphPreprocessor.transformHierarchyEdges(CompoundGraphPreprocessor.java:156)
   at org.eclipse.elk.alg.layered.compound.CompoundGraphPreprocessor.process(CompoundGraphPreprocessor.java:104)
   at org.eclipse.elk.alg.layered.ElkLayered.doCompoundLayout(ElkLayered.java:182)
   at org.eclipse.elk.alg.layered.LayeredLayoutProvider.layout(LayeredLayoutProvider.java:53)
   at org.eclipse.elk.core.RecursiveGraphLayoutEngine.executeAlgorithm(RecursiveGraphLayoutEngine.java:250)
   at org.eclipse.elk.core.RecursiveGraphLayoutEngine.layoutRecursively(RecursiveGraphLayoutEngine.java:218)
   at org.eclipse.elk.core.RecursiveGraphLayoutEngine.layout(RecursiveGraphLayoutEngine.java:96)
   at org.eclipse.elk.core.RecursiveGraphLayoutEngine.layout(RecursiveGraphLayoutEngine.java:72)
   at org.eclipse.elk.core.service.DiagramLayoutEngine.layout(DiagramLayoutEngine.java:703)
   at org.eclipse.elk.core.service.DiagramLayoutEngine.layout(DiagramLayoutEngine.java:631)
   at org.eclipse.elk.core.service.DiagramLayoutEngine$1.execute(DiagramLayoutEngine.java:373)
   at org.eclipse.elk.core.service.util.MonitoredOperation$1.run(MonitoredOperation.java:234)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
   at java.lang.Thread.run(Thread.java:748)

Issue occurs in the following snippet where portPosition has been passed to the method by the caller as null.

                    informationAboutIt = portPosition.y;

The portPosition is only used for fixed order if no port index has been set. However, as the graph does not contain ports, the created dummy ports likely do not have an index.

[soerendomroes]

I don't thing that we can get sensible positions or indices for dummy ports. We could either throw a UnsupportedConfigurationException that states this or try to set a default port position or index.

In other calls of createExternalPortDummy a new KVector() is used as a position instead of null. I suggest to do this also in this case.