Data race while loading builtin domains

[fpoli]

While running multiple verifier instances in parallel (from a Rust program) I reliably get the following error, which might indicate a data race in the way builtin domains are loaded and type-checked.

java.util.NoSuchElementException: None.get
    	at scala.None$.get(Option.scala:627)
    	at scala.None$.get(Option.scala:626)
    	at viper.silicon.supporters.utils$.loadProgramFromUrl(BuiltinDomainsContributor.scala:198)
    	at viper.silicon.supporters.BuiltinDomainsContributor.analyze(BuiltinDomainsContributor.scala:59)
    	at viper.silicon.verifier.DefaultMasterVerifier.$anonfun$analyzeProgramAndEmitPreambleContributions$1(DefaultMasterVerifier.scala:397)
    	at viper.silicon.verifier.DefaultMasterVerifier.$anonfun$analyzeProgramAndEmitPreambleContributions$1$adapted(DefaultMasterVerifier.scala:396)
    	at scala.collection.immutable.List.foreach(List.scala:333)
    	at viper.silicon.verifier.DefaultMasterVerifier.analyzeProgramAndEmitPreambleContributions(DefaultMasterVerifier.scala:396)
    	at viper.silicon.verifier.DefaultMasterVerifier.verify(DefaultMasterVerifier.scala:177)
    	at viper.silicon.Silicon.viper$silicon$Silicon$$runVerifier(Silicon.scala:245)
    	at viper.silicon.Silicon$$anon$1.call(Silicon.scala:198)
    	at viper.silicon.Silicon$$anon$1.call(Silicon.scala:197)
    	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
    	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
    	at java.base/java.lang.Thread.run(Thread.java:832)