FIX: XFRX Previewer generated dangling reference.

KB-20180511-02

Version

Before XFRX 18.1.0

Symptoms

Ramdom cx00000005.
Object of  XFRX#SESSION or of  XFRX#LISTENER didn't release.

Cause

Dandling references in XFXR#PREVIEW and between XFRX#PREVIEW and XFRX#CORE.