Diagnosis for NoSuchElementException from ForkScanner.setHeads

[jglick]

A pipeline (unified-release for @cloudbees employees) had a function like

void skipStage(def stage) {
    println "Skip stage ${stage}"
    org.jenkinsci.plugins.pipeline.modeldefinition.Utils.markStageSkippedForConditional(stage)
}

which I observed to throw an error on one occasion (no idea if reproducible)

java.util.NoSuchElementException
	at java.base/java.util.ArrayDeque.removeFirst(ArrayDeque.java:363)
	at java.base/java.util.ArrayDeque.pop(ArrayDeque.java:594)
	at org.jenkinsci.plugins.workflow.graphanalysis.ForkScanner.setHeads(ForkScanner.java:456)
	at org.jenkinsci.plugins.workflow.graphanalysis.AbstractFlowScanner.setup(AbstractFlowScanner.java:140)
	at org.jenkinsci.plugins.workflow.graphanalysis.AbstractFlowScanner.findFirstMatch(AbstractFlowScanner.java:250)
	at …
	at org.jenkinsci.plugins.pipeline.modeldefinition.Utils.findStageFlowNodes(Utils.groovy:238)
	at org.jenkinsci.plugins.pipeline.modeldefinition.Utils.findStageFlowNodes(Utils.groovy)
	at org.jenkinsci.plugins.pipeline.modeldefinition.Utils$findStageFlowNodes$9.callStatic(Unknown Source)
	at org.jenkinsci.plugins.pipeline.modeldefinition.Utils.markStageWithTag(Utils.groovy:310)
	at org.jenkinsci.plugins.pipeline.modeldefinition.Utils$markStageWithTag$8.callStatic(Unknown Source)
	at org.jenkinsci.plugins.pipeline.modeldefinition.Utils.markStageSkippedForConditional(Utils.groovy:400)
	at org.jenkinsci.plugins.pipeline.modeldefinition.Utils$markStageSkippedForConditional$15.callStatic(Unknown Source)
	at …
	at WorkflowScript.skipStage(WorkflowScript:…)
	at …

It seems the intention was for the return value of leastCommonAncestor to be nonempty, but the assert was not run in production and there is no diagnostic information here.