Uncaught TypeError: Cannot read property 'next' of null

[lisp]

an attempt to layout an apparently innocuous graph leads to that error.
it appears to be a group with a zero length leaves array.
that is, if the first group memer is removed, the layout complete.
otherwise it loops, reiterating the error indefinitely.

the stack trace is

rectangle.js:261 Uncaught TypeError: Cannot read property 'next' of null
at f (rectangle.js:261)
at Object.findXNeighbours [as findNeighbours] (rectangle.js:272)
at generateConstraints (rectangle.js:235)
at generateGroupConstraints (rectangle.js:204)
at rectangle.js:189
at Array.reduce ()
at generateGroupConstraints (rectangle.js:189)
at generateXGroupConstraints (rectangle.js:295)
at Projection.project (rectangle.js:450)
at Projection.xProject (rectangle.js:419)

the graph is

{
"nodes": [
{
"index": 0,
"name": "withSubquery",
"width": 32,
"height": 10
},
{
"index": 1,
"name": "internal",
"width": 32,
"height": 10
},
{
"index": 2,
"name": "withServiceClause",
"width": 32,
"height": 10
},
{
"index": 3,
"name": "asImport",
"width": 32,
"height": 10
},
{
"index": 4,
"name": "1111",
"width": 32,
"height": 10
},
{
"index": 5,
"name": "11",
"width": 32,
"height": 10
},
{
"index": 6,
"name": "1",
"width": 32,
"height": 10
},
{
"index": 7,
"name": "0",
"width": 32,
"height": 10
},
{
"index": 8,
"name": "1111",
"width": 32,
"height": 10
},
{
"index": 9,
"name": "1111",
"width": 32,
"height": 10
},
{
"index": 10,
"name": "1111",
"width": 32,
"height": 10
}
],
"links": [ { "source": 0, "target": 1 } ],
"groups": [
{
"leaves": []
},
{
"leaves": [ 8 ]
},
{
"leaves": [ 9 ]
},
{
"leaves": [ 10 ]
},
{
"leaves": [ 0, 1, 2, 3 ]
}
]
}