fix variant with value type

Author: Myse1f

ic/parser/DIDEmitter.py

@@ -197,7 +197,7 @@ def exitVariantKV(self, ctx:DIDParser.VariantKVContext):
     # Exit a parse tree produced by DIDParser#VariantName.
     def exitVariantName(self, ctx:DIDParser.VariantNameContext):
         key = ctx.Name().getText()
-        self.cache[ctx] = (key, None)
+        self.cache[ctx] = (key, Types.Null)
 
 
     # Exit a parse tree produced by DIDParser#functype.