The Struts 2 conversion error interceptor is a subclass of the XWork 2 conversion error interceptor.
From the Javadocs of the XWork 2 interceptor:
Error formatting macro: snippet: java.lang.NullPointerException
From the Javadocs of the Struts 2 interceptor:
Error formatting macro: snippet: java.lang.NullPointerException
Parameters
Error formatting macro: snippet: java.lang.NullPointerException
Extending the Interceptor
Error formatting macro: snippet: java.lang.NullPointerException
Examples
Error formatting macro: snippet: java.lang.NullPointerException