ruby on rails - can't dup NilClass - Error -

I am stuck in this error for a while and have hit a dead end

Completely got a bad error

  nilclass  

This is not the situation.

I have a class that is in relation to the other

  class guardian end class hair  

The error does not reach the first time

Does this error really cause and is there a solution?

I have referred it but it does not help

update

I found

but this Again gives the same suggestion but I have a module in my script, though there is nothing to do with this model.

Why are you marking the child as unloadable? Is there a good reason for this? If not, then I remove.

Say, "Stability is removed every time a dependency is removed."

What's the error when you change it:

  class hair  

And, I can overflow, but it looks more standard:

  class child belong_to: parental end class mother -Papa has to: Children, Dependent =>: End of Destination