code: def fitNull(self, init_method='emp_cov'):
          
          self.null = self.mtSet1.fitNull(cache=False, factr=self.factr, init_method=init_method)
          self.null['NLL'] = self.null['NLL0']
          self.mtSet2.null = copy.copy(self.null)
          return self.null
docs: fit null model