GtkGrid: Add a few NULL checks
GtkGrid: Add a few NULL checks

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alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,alexlarsson/gtk,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,jigpu/gtk,alexlarsson/gtk,chergert/gtk,davidgumberg/gtk,jadahl/gtk,Adamovskiy/gtk,chergert/gtk,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,davidgumberg/gtk,Adamovskiy/gtk,grubersjoe/adwaita,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,Adamovskiy/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,jadahl/gtk,chergert/gtk,jigpu/gtk,Adamovskiy/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,grubersjoe/adwaita,chergert/gtk