New method checkNull is added to the class to throw a runtime error if mxStream is uninitialized. It is run in the beginning of all methods that require an initialized mxStream. Change-Id: Ia1f15d90c79c71b2a2350d9b60ef1bf68fb9009c Reviewed-on: https://gerrit.libreoffice.org/c/core/+/148819 Tested-by: Jenkins Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>