Since CatalogResolverImpl implements org.w3c.dom.LSResourceResolver - https://docs.oracle.com/en/java/javase/11/docs/api/java.xml/org/w3c/dom/ls/LSResourceResolver.html, its resolveReference method should accept null value for systemId.

LSResourceResolver.resolveResource - https://docs.oracle.com/en/java/javase/11/docs/api/java.xml/org/w3c/dom/ls/LSResourceResolver.html#resolveResource(java.lang.String,java.lang.String,java.lang.String,java.lang.String,java.lang.String) - explicitly states:

{quote}
systemId - The system identifier, a URI reference [IETF RFC 2396], of the external resource being referenced, or null if no system identifier was supplied.
{quote}

but when null systemId is passed to CatalogResolverImpl.resolveResource(...), following NPE is being thrown:

{noformat}
Exception in thread "main" java.lang.NullPointerException: JAXP09020006: The argument 'systemId' can not be null.
at java.xml/javax.xml.catalog.CatalogMessages.reportNPEOnNull(CatalogMessages.java:129)
at java.xml/javax.xml.catalog.CatalogResolverImpl.resolveEntity(CatalogResolverImpl.java:70)
at java.xml/javax.xml.catalog.CatalogResolverImpl.resolveResource(CatalogResolverImpl.java:244)
at java.xml/com.sun.org.apache.xerces.internal.util.DOMEntityResolverWrapper.resolveEntity(DOMEntityResolverWrapper.java:118)
...
{noformat}

and that goes against the definition in LSResourceResolver.resolveResource(), see https://docs.oracle.com/en/java/javase/11/docs/api/java.xml/org/w3c/dom/ls/LSResourceResolver.html#resolveResource(java.lang.String,java.lang.String,java.lang.String,java.lang.String,java.lang.String).

Let me know if test is needed for this.