fix: replace assert with ValueError for llm validation

[iw4p]

It's absolutely better to use raise ValueError instead of assert. (or maybe someone has a better idea)
If you agree for the next refactors I can implement this method

def check_llm(self):
    if self.llm is None:
        raise ValueError("LLM must be set before using this method")

So by using this method whenever we need to use llm, we can easily call it before that line like
self.check_llm()