NPE in ContentModelCompletionParticipant.addCompletionItem

[fbricon]

Given the following xml:

<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd"[
<!ENTITY mdash "&#x2014;">
]>
m| 
<web-app>
  <display-name>Servlet 2.3 Web Application</display-name>
</web-app>

when typing anything outside the nodes, I get an NPE:

[Error - 2:10:18 PM] May 25, 2020 02:10:18 org.eclipse.lemminx.services.XMLCompletions collectOpenTagSuggestions()
Message: While performing ICompletionParticipant#onTagOpen
java.lang.IllegalArgumentException: Property must not be null: label
	at org.eclipse.lsp4j.util.Preconditions.checkNotNull(Preconditions.java:29)
	at org.eclipse.lsp4j.CompletionItem.<init>(CompletionItem.java:144)
	at org.eclipse.lemminx.extensions.contentmodel.participants.ContentModelCompletionParticipant.addCompletionItem(ContentModelCompletionParticipant.java:244)
	at org.eclipse.lemminx.extensions.contentmodel.participants.ContentModelCompletionParticipant.fillWithChildrenElementDeclaration(ContentModelCompletionParticipant.java:183)
	at org.eclipse.lemminx.extensions.contentmodel.participants.ContentModelCompletionParticipant.onTagOpen(ContentModelCompletionParticipant.java:65)
	at org.eclipse.lemminx.services.XMLCompletions.collectOpenTagSuggestions(XMLCompletions.java:608)
	at org.eclipse.lemminx.services.XMLCompletions.collectInsideContent(XMLCompletions.java:733)
	at org.eclipse.lemminx.services.XMLCompletions.doComplete(XMLCompletions.java:224)
	at org.eclipse.lemminx.services.XMLLanguageService.doComplete(XMLLanguageService.java:139)
	at org.eclipse.lemminx.XMLTextDocumentService.lambda$completion$1(XMLTextDocumentService.java:189)
	at java.base/java.util.concurrent.CompletableFuture.biApply(CompletableFuture.java:1307)
	at java.base/java.util.concurrent.CompletableFuture$BiApply.tryFire(CompletableFuture.java:1276)
	at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:479)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1016)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1665)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1598)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)

There's no issue with vscode-xml 0.11.0, so it's a regression