fix: fix backward compatibility

Author: AayushSabharwal

src/index_provider_interface.jl

@@ -126,10 +126,18 @@ end
 Return the observed function of `sym` in `indp` as a [`ParameterObservedFunction`](@ref).
 If `sym` only involves variables from a single parameter timeseries (optionally along
 with non-timeseries parameters) the timeseries index of the parameter timeseries should
-be provided in the `ParameterObservedFunction`. In all other cases, just the observed
-function should be returned as part of the object.
+be provided in the [`ParameterObservedFunction`](@ref). In all other cases, just the
+observed function should be returned as part of the `ParameterObservedFunction` object.
+
+By default, this function returns `nothing`.
 """
-parameter_observed(indp, sym) = parameter_observed(symbolic_container(indp), sym)
+function parameter_observed(indp, sym)
+    if hasmethod(symbolic_container, Tuple{typeof(indp)})
+        return parameter_observed(symbolic_container(indp), sym)
+    else
+        return nothing
+    end
+end
 
 """
     parameter_symbols(indp)

src/parameter_indexing.jl

@@ -301,6 +301,9 @@ function _getp(sys, ::ScalarSymbolic, ::SymbolicTypeTrait, p)
         end
     elseif is_observed(sys, p)
         pofn = parameter_observed(sys, p)
+        if pofn === nothing
+            throw(ArgumentError("Index provider does not support `parameter_observed`; cannot use generate function for $p"))
+        end
         if !is_time_dependent(sys)
             return GetParameterObservedNoTime(pofn.observed_fn)
         end
@@ -624,6 +627,8 @@ function _setp(sys, ::ArraySymbolic, ::SymbolicTypeTrait, p)
     if is_parameter(sys, p)
         idx = parameter_index(sys, p)
         return setp(sys, idx; run_hook = false)
+    elseif is_observed(sys, p) && (pobsfn = parameter_observed(sys, p)) !== nothing
+        return GetParameterObserved{false}(pobsfn.timeseries_idx, pobsfn.observed_fn)
     end
     return setp(sys, collect(p); run_hook = false)
 end