How can I fill null values of one Categorical variable by mode

Question

Here in this data set, there are null values in 'History' columns and there are three category in Age ('Old', 'Young', Middle) in Age columns. I want to replace null values of History column with mode of each History columns based on the Age.

For exapmle:

I want to do like this If Age is 'Old' I want to replace null values of History column with 'Low' as it is mode for History column when Age is Old is Low.

I want to do same, replace null valus of History when Age is 'Middle' with 'High' as the mode of History is 'High' when Age is Midle.

Same thing want do with Age is 'Young' replace History null values with 'Low' as mode of History is 'Low' when Age is 'Young'.

How can I do that?

df.head()
        

         Age    Gender  OwnHome Married Location Salary Children    History Catalogs    AmountSpent
    0   Old     Female  Own     Single  Far      47500    0          High       6         755    
    1   Middle  Male    Rent    Single  Close    63600    0          High       6        1318   
    2   Young   Female  Rent    Single  Close    13500    0          Low       18         296   
    3   Middle  Male    Own     Married Close    85600    1          High      18        2436    
    4   Middle  Female  Own     Single  Close    68400    0          High      12        1304