NullPointerException on EPUB containing invalid links in Navigation Document

[shiestyle]

We found a NullPointerException case when navigation document has an incorrect link.

<nav epub:type="toc" id="toc">
<h1>Navigation</h1>
<ol>
<li><a href="xhtml/p-cover.xhtml">cover</a></li>
<li><a href="xhtml/p-001.xhtml#xxx">p-001</a></li>
</ol>
</nav>

If p-001.xhtml doesn't have an anchor #xxx, NullPointerException will be occurred.

Sorry but Japanese error messages are included.

ERROR(RSC-012): C:\t\20230306-errordata.epub/item/navigation-documents.xhtml(18,37): フラグメント識別子が定義されていません.
java.lang.NullPointerException: Cannot invoke "java.util.List.indexOf(Object)" because the return value of "java.util.Map.get(Object)" is null
	at org.w3c.epubcheck.core.references.ResourceRegistry.getIDPosition(ResourceRegistry.java:57)
	at org.w3c.epubcheck.core.references.ResourceReferencesChecker.checkReadingOrder(ResourceReferencesChecker.java:451)
	at org.w3c.epubcheck.core.references.ResourceReferencesChecker.check(ResourceReferencesChecker.java:105)
	at com.adobe.epubcheck.opf.OPFChecker.checkPackage(OPFChecker.java:149)
	at com.adobe.epubcheck.opf.OPFChecker30.checkPackage(OPFChecker30.java:67)
	at com.adobe.epubcheck.opf.OPFChecker.check(OPFChecker.java:94)
	at com.adobe.epubcheck.ocf.OCFChecker.check(OCFChecker.java:174)
	at com.adobe.epubcheck.api.EpubCheck.doValidate(EpubCheck.java:218)
	at com.adobe.epubcheck.tool.EpubChecker.validateFile(EpubChecker.java:245)
	at com.adobe.epubcheck.tool.EpubChecker.processFile(EpubChecker.java:320)
	at com.adobe.epubcheck.tool.EpubChecker.run(EpubChecker.java:145)
	at com.adobe.epubcheck.tool.Checker.main(Checker.java:31)