Network simplex node placer NoSuchElementException

[soerendomroes]

See https://github.com/kieler/elkjs/issues/183.

Network simplex seems to have a problem with following graph:

algorithm: layered
org.eclipse.elk.partitioning.activate: true
nodePlacement.strategy: NETWORK_SIMPLEX

node n1 {
    layout [size: 30, 30]
    org.eclipse.elk.partitioning.partition: 1
    label "1"
}
node n2 {
    layout [size: 30, 30]
    org.eclipse.elk.partitioning.partition: 2
    label "2"
}
node n3 {
    layout [size: 30, 30]
    org.eclipse.elk.partitioning.partition: 3
    label "3"
}
edge n1 -> n1
edge n2 -> n2
edge n1 -> n2
edge n2 -> n1
edge n2 -> n3

It works with ELK 0.5.0 and fails with higher versions with a NoSuchElementException.