Obviously, vertex indice 424242 does not exist, but test fails, and function
does not return, leading the a crash of the following algorithm (BFS or
Dijkstra).

What did I do wrong?

Thanks for your help.

I would guess that the vertex() function is actually constructing a vertex object or descriptor - not actually trying to access the given vertex. You probably need to use the vertices() function to access the collection and then get the vertex that you need.