No package root found for null

org.eclipse.core.runtime.CoreException: No package root found for null
at com.microsoft.jdtls.ext.core.PackageCommand.getPackageRootChildren(PackageCommand.java:370)
at com.microsoft.jdtls.ext.core.PackageCommand.getChildren(PackageCommand.java:109)
at com.microsoft.jdtls.ext.core.CommandHandler.executeCommand(CommandHandler.java:31)
at org.eclipse.jdt.ls.core.internal.handlers.WorkspaceExecuteCommandHandler$1.run(WorkspaceExecuteCommandHandler.java:230)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
at org.eclipse.jdt.ls.core.internal.handlers.WorkspaceExecuteCommandHandler.executeCommand(WorkspaceExecuteCommandHandler.java:220)
at org.eclipse.jdt.ls.core.internal.handlers.JDTLanguageServer.lambda$4(JDTLanguageServer.java:604)
at org.eclipse.jdt.ls.core.internal.BaseJDTLanguageServer.lambda$0(BaseJDTLanguageServer.java:87)
at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(Unknown Source)
at java.base/java.util.concurrent.CompletableFuture$Completion.exec(Unknown Source)
at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(Unknown Source)
at java.base/java.util.concurrent.ForkJoinPool.scan(Unknown Source)
at java.base/java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
at java.base/java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)

Repro steps:

1. Open https://github.com/microsoft/build-server-for-gradle
2. Expand
   

PackageCommand.java の PackageCommand.getPackageRootChildren から開始し、スタックトレースに示されている getChildren コマンドの経路を追跡します。build-server-for-gradle を開いてプロジェクトツリーを展開し、再現します。展開時に「No package root found for null」が報告されなくなれば完了です。