Yup, this. The King of Rome was Napoleon's son. If he doesn't exist in this TL, Joseph inherits according to the constitution -- although more likely the Empire just collapses. Certainly Louis Bonaparte, who was spending his time writing poetry in Graz as a guest of Emperor Franz, is a highly implausible candidate to usurp the throne.
The Prince Imperial was born in 1811, so presumptively he is alive and now the titular Emperor of the French.