My dog won't go outside at night alone.

Abbey is a 2 1/2 year old female chihuahua/Cavoodle. She is very sociable and loves all visitors, greeting them always with one of her toys. Has a doggy door and uses it always during the day to go outside. But will not go outside at night unless I go with her. I always take her out before bed at about 9.30pm and if I’m up during the night she, too, will stand at her doggy door waiting for me to take her out. I always have to stand and watch while she finds a spot and does a wee. If I don’t stay there sh comes straight inside again. If I happen to sleep through the night I will wake to find a puddle either on the carpet or on the vinyl just inside her doggy door. She seems to be very nervous outside and jumps if a slight breeze moves any of the plants in the garden. But during the day is happy to be out there and just lie down in the sunshine. How do I change this fear of the dark?
PS. My husband passed away 6 months ago and I put her on to PAW Complete Calm (Blackmores) when I realised how much she was grieving and this brought back her mojo!