Error "Index was outside the bounds of the array" for EDMX

Question

OK. When I'm actually trying to Update the modal, some additional keys are getting added.

For solving that, I've opened the edmx file with xml editor. then removed the additional keys. Then the edmx build successfully. But the problem is when I do 'validate' the edmx, then visual studio throws strange error "Index was outside the bounds of the array". Note: this also comes at runtime.

Any ideas why?