Add crosslink restraint to specific state

[ichem001]

General: It would be useful if we could add a crosslink restraint to a specific state rather than to the whole system; that would add extra control on what each state is trying to optimize crosslinking-wise.
I tried to pass the following objects to the restraint:
-representation.get_children()[1]
-bs.system.get_states()[1]
-bs.system.get_states()[1].get_hierarchy()

Excluded Volume and Connectivity restraints can be set up for "State 0" and "State 1" separately without problem.

Specific question: Interestingly, the restraint is created and set up when using index [0] instead of index [1] ("State 0" vs "State 1"). I don't quite understand why "State 1" is not setting up correctly here.
For "State 0" the defaultdict shows all the molecules but for "State 1" the defaultdict is empty.
Maybe it is the way the second state is built for which something is missing ---