Example simple paniced

[LinPr]

I cloned the newest master branch and wanted to run some tests. However, I was confused when I ran /example/simple.go, as the program just panicked.

It seems this example doesn't work as I expected. Can anyone please explain to me how the subflow and main taskflow work together? Can I recognize the entire subflow as a normal process graph node of the main taskflow DAG? Inside the subflow, does it work just like an independent DAG taskflow?

A
B1
C1
A1
2025/03/31 15:14:16 graph G is canceled, since subflow sub1 panics
C
2025/03/31 15:14:16 [recovered] subflow sub1, panic: Taskflow is frozen, cannot new tasks, stack: goroutine 23 [running]:
runtime/debug.Stack()
        /usr/local/go/src/runtime/debug/stack.go:26 +0x5e
github.com/noneback/go-taskflow.(*innerExecutorImpl).invokeNode.(*innerExecutorImpl).invokeSubflow.func2.1()
        /home/lin/C++42/Go/taskflow/go-taskflow/executor.go:143 +0x10a
panic({0x652ac0?, 0x6bbe70?})
        /usr/local/go/src/runtime/panic.go:792 +0x132
github.com/noneback/go-taskflow.(*TaskFlow).push(...)
        /home/lin/C++42/Go/taskflow/go-taskflow/taskflow.go:28
github.com/noneback/go-taskflow.(*TaskFlow).NewTask(0xc0001260e0, {0x687c29?, 0xc0001341a0?}, 0x693448)
        /home/lin/C++42/Go/taskflow/go-taskflow/taskflow.go:45 +0x245
main.main.func7(0x534037?)
        /home/lin/C++42/Go/taskflow/go-taskflow/examples/simple/simple.go:47 +0x3b
github.com/noneback/go-taskflow.(*innerExecutorImpl).invokeNode.(*innerExecutorImpl).invokeSubflow.func2()
        /home/lin/C++42/Go/taskflow/go-taskflow/executor.go:164 +0x1b0
github.com/noneback/go-taskflow/utils.(*Copool).CtxGo.func1()
        /home/lin/C++42/Go/taskflow/go-taskflow/utils/copool.go:70 +0x7c
github.com/noneback/go-taskflow/utils.(*Copool).CtxGo.func2()
        /home/lin/C++42/Go/taskflow/go-taskflow/utils/copool.go:93 +0xf3
created by github.com/noneback/go-taskflow/utils.(*Copool).CtxGo in goroutine 1
        /home/lin/C++42/Go/taskflow/go-taskflow/utils/copool.go:81 +0x1d6
2025/03/31 15:14:16 graph G canceled, since subgraph sub1 is canceled
2025/03/31 15:14:16 node B is not scheduled, since graph G is canceled

[noneback]

Thanks for your pr. I left some comments, and you can try to revise in your spare time.