Installation Failed “Invalid argument” when trying to run Today application extension

Question

I added a vanilla Today Application Extension and run with the new scheme created by XCode.
After it finishes building (\"Build Succeeded\" pops up), it failed straight

Answer 1

I was getting the same error - although in different conditions - and it took me a while to realise that I had inadvertently messed with the Info.plist (thank you user2866588!) via a Search & Replace operation.

Once fixed that file, the error went away. It's a long shot, but you never know...