code: @Nullable
      private String isFieldCollection(OpcodeStack.Item item) throws ClassNotFoundException {
          Comparable<?> aliasReg = (Comparable<?>) item.getUserValue();
          if (aliasReg instanceof String) {
              return (String) aliasReg;
          }
  
          XField field = item.getXField();
          if (field == null) {
              return null;
          }
  
          JavaClass cls = item.getJavaClass();
          if ((cls != null) && cls.implementationOf(collectionClass)) {
              return field.getName();
          }
  
          return null;
      }
docs: determines if the stack item refers to a collection that is stored in a field
  
  @param item
  the stack item to check
  
  @return the field name of the collection, or null
  @throws ClassNotFoundException
  if the items class cannot be found