For the UEFA Champions League, each round has a separate draw. In the group round and the first knock out round, the draw is made in such a way that no two teams from any league meet. Real Madrid and Barcelona, both play in La Liga. What are the chances of a Real Madrid-Barcelona final in the UEFA Champions League?

[Assume they were in the top $4$ in La Liga, meaning they qualified directly for the group stage of the UEFA Champions League]