Explicit null check
Explicit null check

Seen this cause JunitTest_sc_unoapi once with a NullPointerException, though I
do not know whether ac can legitimately be null here.

Change-Id: I1ca40dfca2d1a597842fba011a813be8154d8dd8

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core