if else logic with empty dates

I'm trying to write a function that returns some dates in python.

def set_deadline(row):
    if row["date1"] is not None:
        return row["date1"]+ relativedelta(days=+90)
    else:
        return None

since some of the records have the date1 field missing (NaT), the error " bad month number nan; must be 1-12" shows up why I try to use this function on my dataframe. How can I change the function so that the deadline would be Nan or None if the date1 field is missing?

I also tried just to define the deadline field by doing the following:

df['Deadline']= df['date1']+ relativedelta(days=+90)

but I got the error, "incompatible type [object] for a datetime/timedelta operation"