Fixes: NB#268062 - Segmentation fault when systemwide sheet's statusbar visibility state is changed during sheet is appearing.
Fixes: NB#268062 - Segmentation fault when systemwide sheet's statusbar visibility state is changed during sheet is appearing.

RevBy: Stanislav Ionascu
MR: 2039

Details:
Checks  the sheet pointer before using it in order to prevent eventual
crashes.

nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch,nemomobile-graveyard/libmeegotouch