FIXED_SIDE and FIXED_POS ports throw exception when defined on hierarchical node of 2 levels of nesting

[alexei-tilikin]

Using thelayered algo with elkjs. The graph has deep nesting of nodes inside nodes. Problem reproduces on [email protected] and [email protected]

On level 2 root.children[*].children[*].layoutData, if I define the option 'elk.portConstraints': 'FIXED_POS' or 'elk.portConstraints': 'FIXED_ORDER', elk will throw exception:

TypeError: Cannot read properties of null (reading 'x_0')
    at createExternalPortDummy (elk-worker.js:31762:1)
    at $transformHierarchyEdges (elk-worker.js:30937:1)
    at $transformHierarchyEdges (elk-worker.js:30927:1)
    at $process_1 (elk-worker.js:30774:1)
    at $doCompoundLayout (elk-worker.js:27705:1)
    at LayeredLayoutProvider.layout_2 [as layout] (elk-worker.js:28027:1)
    at $executeAlgorithm (elk-worker.js:61782:1)
    at $layoutRecursively (elk-worker.js:61872:1)
    at $layout_3 (elk-worker.js:61817:1)
    at layout_11 (elk-worker.js:74016:1)
    at Dispatcher.dispatch (elk-worker.js:73945:1)
    at Dispatcher.saveDispatch (elk-worker.js:73953:1)
    at elk-worker.js:73970:1

Here's the demo reproducing the problem

The problem reproduces after adding the option on group_1 but not when it's on group_2.

More info is in the discussion here: https://github.com/kieler/elkjs/issues/162