I have StartDate and ExitDate two columns in my dataframe with NaT values in ExitDate column

I wish to create a third column Tenure by finding Difference between ExitDate and StartDate.

StartDate   ExitDate
2022-06-22  2022-08-07
2020-12-28  NaT
2020-12-09  2022-08-04
2019-05-28  2021-10-23
2022-04-27  NaT

The below code gives error
‘ufunc ‘isnat’ is only defined for np.datetime64 and np.timedelta64.’
Although the datatypes are

 def calculateTenure(row):
    if np.isnat(row.ExitDate):
        ExitDate = pd.datetime.now()
    return (row.ExitDate.year - row.StartDate.year)
        
df['Tenure'] = df.apply(lambda row:calculateTenure(row), axis=1)

The main motive is to check if ExitDate is NaT so that current date can be used for calculation instead.