Forum:Chain problem: Heavy Storm, then Starlight Road, then Mystical Space Typhoon

My opponent activates Heavy Storm, then I activate Starlight Road to negate it and destroy it. Then, my opponent decides to activate Mystical Space Typhoon from his hand to destroy Heavy Storm, a card he already activated.

He says I can't Special Summon Stardust Dragon because of it, but MST does not negate effects; it only destroys Spells/Traps on the field. How does destroying Heavy Storm, a non-continous spell he already activated, prevent me from triggering the second effect of my Starlight Road?