An error will be reported when running the example

When loading "BehaviorTree.CPP/examples/test_files/subtree_test.xml" in Groot, the following error will appear? What is the reason and how to solve it?

![image](https://user-images.githubusercontent.com/77045200/224600971-462eb593-ac04-4916-9178-b938311131fc.png)

![image](https://user-images.githubusercontent.com/77045200/224601054-2f7f5813-374a-4e16-8f51-dab443dc38d1.png)
