In order to build Starship Defenses, you should first have to research Starship Weapons. Also, in order to start a "defense theory" track, you should have to complete the "weapon theory" track first. For example, how can I research "Shield Defense Theory" if I haven't even theorized what a Beam Weapon is? That is like mankind developing Kevlar body armor before gunpower and muskets were discovered. This should be a pretty minor effort to change in the tech tree, but I think it is important.