I don't see a problem with A. And B is correct too. It's just that the focus is different. A focuses on an ongoing action very close to the moment of speech, whereas B includes a longer span of time and can be interpreted as a simple future.
I imagine A being said in an environment where a task is being carried out, and immediate assistance is offered in case of need.
I understand B said in an identical situation as A, but it is not limited to it. It is also possible when someone carries out a task in an unspecified time and until that task comes to an end, assistance is offered in the future.
This site states:
In the condition clause we can use present continuous:
- If I’m driving, I don’t talk on my phone.
A language blog says:
Imperatives with the zero conditional
We often use the zero conditional to tell people what to do, or to give instructions. To do this, we use the structure: If / When + present tense, imperative
- When the water is boiling, add the pasta.