ClassCastException: with documentLink in catalog XMLCatalogDocumentLinkParticipant

[angelozerr]

Open the given XML file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE catalog PUBLIC "-//OASIS//DTD XML Catalogs V1.1//EN" "https://www.oasis-open.org/committees/entity/release/1.1/catalog.dtd" >
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
    <!-- W3C schema -->
 
    <uri name="urn:speedata.de:2009/publisher/en" uri="layoutschema-en.xsd"/>
 
</catalog>

The document link doesn't work for the uri attribute and you can see the error message:

[Error - 18:33:14] Request textDocument/documentLink failed.
  Message: Internal error.
  Code: -32603 
java.util.concurrent.CompletionException: java.lang.ClassCastException: class org.eclipse.lemminx.dom.DOMDocumentType cannot be cast to class org.eclipse.lemminx.dom.DOMElement (org.eclipse.lemminx.dom.DOMDocumentType and org.eclipse.lemminx.dom.DOMElement are in unnamed module of loader 'app')
	at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314)
	at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319)
	at java.base/java.util.concurrent.CompletableFuture.biApply(CompletableFuture.java:1238)
	at java.base/java.util.concurrent.CompletableFuture$BiApply.tryFire(CompletableFuture.java:1205)
	at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:479)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)
Caused by: java.lang.ClassCastException: class org.eclipse.lemminx.dom.DOMDocumentType cannot be cast to class org.eclipse.lemminx.dom.DOMElement (org.eclipse.lemminx.dom.DOMDocumentType and org.eclipse.lemminx.dom.DOMElement are in unnamed module of loader 'app')
	at org.eclipse.lemminx.extensions.catalog.CatalogUtils.getCatalogEntries(CatalogUtils.java:67)
	at org.eclipse.lemminx.extensions.catalog.XMLCatalogDocumentLinkParticipant.findDocumentLinks(XMLCatalogDocumentLinkParticipant.java:36)
	at org.eclipse.lemminx.services.XMLDocumentLink.findDocumentLinks(XMLDocumentLink.java:38)
	at org.eclipse.lemminx.services.XMLLanguageService.findDocumentLinks(XMLLanguageService.java:232)
	at org.eclipse.lemminx.XMLTextDocumentService.lambda$documentLink$12(XMLTextDocumentService.java:363)
	at java.base/java.util.concurrent.CompletableFuture.biApply(CompletableFuture.java:1236)
	... 7 more

If you remove DOCTYPE, it works good.

To fix theproblem we should check the node is an element https://github.com/eclipse/lemminx/blob/dbf43f3cae27a726c22e319eca939872c83fb3d8/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/catalog/CatalogUtils.java#L66