After installing Xcode 7 I can't build and run projects anymore as document fails to open

Question

EDIT: This question refers to teh pre-release version of Xcode 7, hence it may not be relevant if you are running the currently released version as some issues may have been

Answer 1

I am not sure which answer to accept as for me what worked was installing a later beta (beta 5). I had filed a bug to Apple a couple of months ago so I assume they solved this problem by updating Xcode.

Anyone experiencing this with the current official release?

I post this as answer for now but if anyone has a more detailed explanation I will be happy to remove my answer and accept yours.