code: def count_nulls(self, field):
          
          try:
              n = self.df[field].isnull().sum()
          except KeyError:
              self.warning("Can not find column", field)
              return
          except Exception as e:
              self.err(e, "Can not count nulls")
              return
          self.ok("Found", n, "nulls in column", field)
docs: Count the number of null values in a column