error in random forest analysis
This question will be a little hard to answer without more information.
For example, we will need to see your dataset (whole_root_pred), to decide why Stunting_Root is NULL.

You might need to initialize Stunting_Root as a variable. It is currently not clear if it is e.g. a column of your dataframe, or just uninitialized. Uninitialized variables are NULL, which would explain your problem. randomForest might not know to look for strata inside your dataframe, for example. Is it in your dataframe?
Also, I might be missing something, but why are you passing ? as a response? I'm not an expert but I believe that is an illegal character in R (I'm pretty sure?).