Xcode - Invalid character in source file (Replace " " with " ")

I have a question about Xcode (Swift). Sometimes when I write an if statement. Something like this:

if(a == 2){
//do Sth
} else {
//do Sth
}

Xcode throws me an error: "Invalid character in source file" I know it is no big deal and fixed by clicking on "Fix-it" but I want to know what is the problem. That doesn't make sense to me...

My Xcode is on 7.2 and I am using Swift - I don't know if this is just a Swift problem.