replaceItemOfList by assigning array index

src/blocks/scratch3_data.js

@@ -180,7 +180,7 @@ class Scratch3DataBlocks {
         if (index === Cast.LIST_INVALID) {
             return;
         }
-        list.value.splice(index - 1, 1, item);
+        list.value[index - 1] = item;
         list._monitorUpToDate = false;
     }