Talk to her about her bad dreams. She's 'lost' one parent, remember and probably doesn't understand why. You might want to take her to a kids' counselor to explore some of her fears and adjustment issues. Make sure you spend extra time with her during the evening - read to her before bed, or develop other night time rituals. She needs some extra loving to reassure her that she'll be taken care of.
Kids sometimes worry a lot about being left alone, especially about moms.
Let her know that she is welcome to sleep in your room when she can't get back to sleep in her own bed, but she will have to sleep on the floor. Put a blanket and pillow on the floor for her and let her use that in the middle of the night. Stick with making her go to sleep in her own bed, though, she can only come in when she wakes up at night. Then, after a week or so, tell her that you will limit the number of nights she can sleep in your room. On the nights that she isn't allowed in, lock your door or put up a folding gate.