fix: fix discrete variable detection in IndexCache

Author: vyudu

src/systems/index_cache.jl

@@ -117,10 +117,11 @@ function IndexCache(sys::AbstractSystem)
             affs = [affs]
         end
         for affect in affs
-            if affect isa Equation
-                is_parameter(sys, affect.lhs) && push!(discs, affect.lhs)
-            elseif affect isa FunctionalAffect || affect isa ImperativeAffect
+            if affect isa AffectSystem || affect isa FunctionalAffect ||
+               affect isa ImperativeAffect
                 union!(discs, unwrap.(discretes(affect)))
+            elseif isnothing(affect)
+                continue
             else
                 error("Unhandled affect type $(typeof(affect))")
             end