NPE in XMLCompletions collectAttributeNameSuggestions()

[fbricon]

Given:

<!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;">
]>
<web-app a|>
  <display-name>Servlet 2.3 aWeb Application</display-name>
</web-app>

when typing an attribute in the web node, an NPE is thrown in XMLCompletions collectAttributeNameSuggestions():

[Error - 2:17:54 p.m.] May 25, 2020 02:17:54 org.eclipse.lemminx.services.XMLCompletions collectAttributeNameSuggestions()
Message: While performing ICompletionParticipant#onAttributeName
java.lang.NullPointerException
	at org.eclipse.lemminx.extensions.dtd.contentmodel.CMDTDDocument.findElementDeclaration(CMDTDDocument.java:164)
	at org.eclipse.lemminx.extensions.dtd.contentmodel.CMDTDDocument.findCMElement(CMDTDDocument.java:151)
	at org.eclipse.lemminx.extensions.contentmodel.model.ContentModelManager.findInternalCMElement(ContentModelManager.java:204)
	at org.eclipse.lemminx.extensions.contentmodel.model.ContentModelManager.findInternalCMElement(ContentModelManager.java:191)
	at org.eclipse.lemminx.extensions.contentmodel.participants.ContentModelCompletionParticipant.onAttributeName(ContentModelCompletionParticipant.java:263)
	at org.eclipse.lemminx.services.XMLCompletions.collectAttributeNameSuggestions(XMLCompletions.java:789)
	at org.eclipse.lemminx.services.XMLCompletions.doComplete(XMLCompletions.java:120)
	at org.eclipse.lemminx.services.XMLLanguageService.doComplete(XMLLanguageService.java:138)
	at org.eclipse.lemminx.XMLTextDocumentService.lambda$completion$1(XMLTextDocumentService.java:177)
	at java.base/java.util.concurrent.CompletableFuture.biApply(CompletableFuture.java:1307)
	at java.base/java.util.concurrent.CompletableFuture$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)

Occurs in vscode-xml 0.11.0:

[fbricon]

The bug seems to be caused by the [<!ENTITY mdash "&#x2014;">] body of the DOCTYPE. It doesn't occur without it.

[angelozerr]

The bug seems to be caused by the [] body of the DOCTYPE. It doesn't occur without it.

Indeed there were a bug with Doctype subset (when it is not empty) but which have no ELEMENT declaration. The bug was that it created one CMElementDeclaration whith a null name. The PR #721 fixes this problem (no CMElementDeclaration are created when there are no ELEMENT declaration). This PR fixes all problem that we could have with CMElementDeclaration#getName() which could be null like #719 and #720

In otherwords this issue was fixed but I have written a test with this usecase in the PR #721