Is there as named phobia where you fear that your husband will die before you and leave you destitute?

It's not a specific phobia, but I believe you are talking about form of fear of abandonment. Sometimes people become highly dependent upon others and they don't know what they would do without their partner. But while "I don't know what I would do without you" is a common sentiment, this is a fear that creates abnormal behaviors. This is sometimes related to agoraphobia, when people fear going out in public without someone they can trust. It can also be related to borderline personality disorder, though more so in the fear of someone leaving you of their free will.