up for mtk parent change

Author: TorkelE

src/spatial_reaction_systems/lattice_reaction_systems.jl

@@ -117,7 +117,7 @@ struct LatticeReactionSystem{Q, R, S, T} <: MT.AbstractTimeDependentSystem
         if !isempty(MT.continuous_events(rs)) || !isempty(MT.discrete_events(rs))
             throw(ArgumentError("The `ReactionSystem` used as input to `LatticeReactionSystem` contain events. These will be ignored in any simulations based on the created `LatticeReactionSystem`."))
         end
-        if !isempty(MT.get_parent(rs))
+        if !isnothing(MT.get_parent(rs)) && !isempty(MT.get_systems(MT.get_parent(rs)))
             @warn "The `ReactionSystem` used as input to `LatticeReactionSystem` was originally created as a hierarchical model. While this won't necessarily result in errors, it has not been well-tested, and is not recommended."
         end
         if !isempty(observed(rs))