Arguing the constitution is invalid is effectively arguing for anarchy.  50 years ago there was a horrible civil war in Spain.  Make no mistakes, we have no idea how many Catalonians want out of Spain (the referendum was not participated by the NO side since it was deemed illegal) and we have no idea if they want out with the economic repercussions of being out of the EU.

Either way, they need an new election in Catalonia and then possibly a referendum with clarity on the choices.  

If not, you will see civil war in Barcelona and that will be the tragedy.