code: @Override
      public void visitClassContext(ClassContext clsContext) {
          try {
              if ((collectionCls == null) || (setCls == null) || (mapCls == null)) {
                  return;
              }
  
              stack = new OpcodeStack();
              super.visitClassContext(clsContext);
          } finally {
              stack = null;
          }
      }
docs: implement the visitor to set up the opcode stack, and make sure that collection, set and map classes could be loaded.
  
  @param clsContext
  the context object of the currently parsed class