|   8.2.1 relative pronouns referencing nouns | 
If we talk about relative pronouns which refer a noun of the main clause we talk about something like that. 
The jacket (that) I bought yesterday was expensive.  
We built a house on whose roof is a meadow.  | 
                      
                        In this case we don't talk about a relative pronoun which refers to an idea, we don' t talk about something like that. 
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He lost his job what was really bad, because for this reason he couldn' t pay the rent.  | 
What refers to the whole sentence, to an idea, not to a noun.