A "true" allergy is a Type 1 IgE mediated hypersensitivity in which antibodies recognize specific antigens. Antibodies are created when an interplay between T cells and B cells occurs, notably the presentation of a protein on a Major HistoCompatability (MHC) complex, which induces the creation and release of antibodies that the target that protein. As MHC complexes only recognize proteins, a "true" allergy ONLY can happen with a protein. However, fruit has protein in it as well, as do most food items other than pure sugar or pure fat, so it is not impossible. I have many children in my clinic allergic to kiwi. However you could not be allergic to pure table sugar or pure salt, for the reason above (however pure substances dont generally exist outside of the lab and there is almost always some plant matter contaminant)
Nickle "allergy" is a delayed type 4 cell mediated hypersensitivity, in which the nickle ions act as a hapten. A hapten is a molecule that combines with a protein to form larger allergenic molecule. The ion binds with a protein and that protein is recognized by dendritic cells in the skin that cause an allergic type reaction. So it IS a protein that is being recognized, but a different type of mechanism and really not what we mean when we say a "true allergy".
Thus "true" allergies, type 1 hypersensitivities, are caused by only proteins which are omnipresent, but even exposure to non proteins can cause allergic type reactions when they combine with the bodies own proteins to form an allergenic molecule.