If you save the map during the interactive room, placing the sentry gun\seeds\filling the incubator, the script for detecting that you have done the task will reset.
To bypass the bug simply pretend to do the part again.
Hover a new gun and ammo next to your already placed one, hover a seed over each seed bed, take the alien egg out then put it back in the incubator and finally hover a new light infront of the already installed one.
Doing this should open the door and allow you to continue the map.

You could "ghost" though the door but you'll suffer a big hit to your score, you're better off doing the above.