fix: handle problems with no system in SavedSubsystem constructor

src/solutions/save_idxs.jl

@@ -44,6 +44,11 @@ function as_diffeq_array(vt::Vector{VectorTemplate}, t)
     return DiffEqArray(typeof(TupleOfArraysWrapper(vt))[], t, (1, 1))
 end
 
+function is_empty_indp(indp)
+    isempty(variable_symbols(indp)) && isempty(parameter_symbols(indp)) &&
+        isempty(independent_variable_symbols(indp))
+end
+
 """
     $(TYPEDSIGNATURES)
 
@@ -104,6 +109,12 @@ function SavedSubsystem(indp, pobj, saved_idxs)
         return nothing
     end
 
+    # this is required because problems with no system have an empty `SymbolCache`
+    # as their symbolic container.
+    if is_empty_indp(indp)
+        return nothing
+    end
+
     # array state symbolics must be scalarized
     saved_idxs = collect(Iterators.flatten(map(saved_idxs) do sym
         if symbolic_type(sym) == NotSymbolic()