A NPE occurs when exporting a xslx as an XML document if the attached schema contains ref elements. 

Here is an example of a xml schema which POI can't handle beacause of the xs:element ref attribute

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
    <xs:element name="Entry" type="Type_Entry"/>
    <xs:complexType name="Type_Entry">
        <xs:sequence>
             <xs:element ref="REFELEMENT" minOccurs="0"/>
        </xs:sequence>
    </xs:complexType>
    
    <xs:element name="REFELEMENT" type="xs:double"/>

</xs:schema>

I'm going to provide a patch for this issue.