If an element in a stylesheet is in a namespace marked
as an extension element namespace, and Saxon does not
recognize the namespace, and the instruction has an
xs:fallback child, then the xsl:fallback child is not
properly validated before being compiled. This could
cause a wide variety of possible symptoms; the one that
was reported was a NullPointerException in
Applies to 7.9.1 and probably to most previous releases.