<human>:  Give me a Python program for filling null value sin data frame.
<bot>:})

# Calling the function
df = fill_null_value(df)

# Printing the dataframe
print(df)

#Example Output

#  a b
# 0 1 1
# 1 2 2
# 2 3 3
# 3 4 4
# 4 5 5

#Remember to inspect code for bugs.