Prior to rB608d9b5aa1f1, the constraint was gotten using context [CTX_data_pointer_get_type(C, "constraint", &RNA_Constraint) -- which is valid for bones on hidden layers].
After rB608d9b5aa1f1, the constraint is found (or isnt) using edit_constraint_property_get [this is not valid for bones on hidden layers because internally BKE_pose_channel_active checks if the bone is on an active layer].
- Every operator using edit_constraint_property_get doesnt work for bones on inactive layers [delete, moveup, movedown, move to index (drag n drop nowadays)]
- moveup, movedown, move to index check if they could find a constraint beforehand though (dont crash)
- delete crashes (doesnt check if a constraint could actually be found)
- Every operator using edit_constraint_property_get for constraint data doesnt work for bones on inactive layers [stretchto_reset, limitdistance_reset, childof_set_inverse, ...]
- these all check if they could find a constraint beforehand though (dont crash)
This is because the poll function is using context to get the constraint, the operators themselves use edit_constraint_property_get which leads to inconsistent/unexpected results.
Possible solutions were:
-  let the delete operator just work with the context constraint again (like prior to rB608d9b5aa1f1) -- allows for deleting constraints on bones in inactive layers
-  check if we could get a constraint -- prevents the crash, but does not allow for deleting constraints on bones in inactive layers
-  make the poll edit_constraint_poll_generic be as strict as the operators -- dont use context to get the constraint, but something like edit_constraint_property_get
-  make the operators be more graceful and let them act on bones on hidden layers -- let edit_constraint_property_get actually use context
Whatever we decide upon: this might be a candidate for 2.90.1?
This patch implements , so poll and operators are now in sync.
- prevents reported crash
- also enables operators for bone constraints on hidden layers
- also enables drag and drop reordering of constraints on hidden layers
Note: Adding constraints also doesnt work for bones on inactive layers [that was the case in 2.79 as well -- it is also using BKE_pose_channel_active]
Note: edit_constraint_invoke_properties also uses said context