How to convert the null value from the specific field.

Hi,

I am attempting to converto Null value to specific fields value.
Though I had watched the discussion as following, I couldn't convert the field's data

https://community.alteryx.com/t5/Data-Preparation-Blending/Multi-Field-Tool-with-dynamic-fieldnames-in-formula-expression/m-p/42749#M7933

What I'd like to do is to convert it to data in another field if particular field contains Null value.

For example, in the 2nd line of the table, convert the value of Field_4(null) into the value of r_Avg_Field_4.