A DESCRIPTION OF THE PROBLEM :
According to the JavaDoc 11 it is allowed to use null as argument to XMLStreamWriter.setDefaultNamespace():
"Parameters:
    uri - the uri to bind to the default namespace, may be null"

Since Java 8 the implementation within the subsequently called method 'isDefaultNamespace()' changed from:

    if (uri == defaultNamespace) {
        return true;
    }

to:

    if (uri.equals(defaultNamespace)) {
        return true;
    }

In cases where 'uri' is null the call of 'equals()' leads to a NullPointerException.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
XMLOutputFactory outFactory = XMLOutputFactory.newFactory();
outFactory.setProperty(XMLOutputFactory.IS_REPAIRING_NAMESPACES, new Boolean(true));
BufferedOutputStream bufferedStream = new BufferedOutputStream(outStream);
XMLStreamWriter xsw = outFactory.createXMLStreamWriter(bufferedStream, "UTF-8");
xsw.setDefaultNamespace(null);


ACTUAL -
Caused by: java.lang.NullPointerException
        at java.xml/com.sun.xml.internal.stream.writers.XMLStreamWriterImpl.isDefaultNamespace(XMLStreamWriterImpl.java:1733)
        at java.xml/com.sun.xml.internal.stream.writers.XMLStreamWriterImpl.setDefaultNamespace(XMLStreamWriterImpl.java:458)