No worries. Your English is better than any of my second languages etc., that's for sure.
I was just defining what a void contract is. It wouldn't arise here, because simply hiring a third party to carry out ostensibly the same function doesn't render a contract void. Likewise, the party who might be seeking to void the contract would be making a financial loss by having it declared void. It's more likely that one of the parties would consider the contract, which they would continue to be bound by until a decision of a Court, to be frustrated and therefore seek damages on foot of their alleged losses.
In reality it won't happen, though, as at the appropriate time they would simply terminate the contract of the current manager) leaving the other one to carry on. Similarly everyone knows how football works. You rarely get the sack without the next man being in place. There is no prescriptive job description or title that clubs are bound by when hiring managers. The mere fact that you've entered into a contract based on future events doesn't mean that that agreement is void because you already have a manager. It could mean, though, that if the manager was undermined that he could argue constructive dismissal etc. That wouldn't be at all common. Generally it's just about sorting a payoff.