Watching the replay of the goal, for me Johnstone isn't at fault for that goal, I don't think he can get to that cross. Granted his positioning probably could have been better but you cant place the blame on him.
The players back post should be dealing with things better, think it's HRK who switches off before reacting late to the danger.
I think Johnstone does get a lot of stick, most unjustified in all honesty. I think the way the team set up last year meant he was quite exposed, playing out from the back etc and a number of fans made their mind up that he wasn't good enough.
We need to remember we were spoilt by having Foster in goal for as long as we did. For me Johnstone is on of the best keepers in this league, I could probably only count on one hand the keepers in this league I would prefer in goal instead of him.
The constant criticism of him on twitter and instagram is unwarranted, and don't be surprised if that affects a players performance.