Why was the apostles' question about restoring the kingdom to Israel a valid one?
It was valid because for forty days after His resurrection, Jesus spoke to the apostles about the things pertaining to the Kingdom of God. Acts 1:3 says He was seen forty days and spoke of the Kingdom. Their question fit that sustained emphasis: the Kingdom is the Kingdom, one and the same.
Les explains that Acts 1:3 makes the apostles' question valid because for the forty days after His passion, Jesus was speaking to them about the things pertaining to the Kingdom of God. Les emphasizes, "the Kingdom is the Kingdom is the Kingdom"; it is one and the same. So the eleven men were focused on the Kingdom. Les also points to Matthew 19 to show that this Kingdom cannot take place in heaven, has never taken place before, and so still has to be future. That context explains why their question about the Kingdom was not out of place.
To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God: