Adding the null check fixes a bug that made wireshark crash
when right clicking a subtree which doesn't have any preferences
and if none of the parents had one either.
The problem was introduced in commit 589413d877
Change-Id: Ia5bbae0a58298f3e9d912e44f33589da1cbfacc9
Reviewed-on: https://code.wireshark.org/review/29455
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Petri Dish Buildbot
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>