Does target selection happen when a card is added to the chain or when it resolves?

Ruling: Target selection happens when a card is added to the chain. You cannot add a card to the chain if it doesn't have a valid target.

Nuances:

• Cards like Icathian Rain and Falling Star are exceptions that target on resolution, not on cast
• These exception cards create multiple new triggers (6 for Icathian Rain, 2 for Falling Star) that get added to the chain and require targets at that point