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I recently upgraded from Xcode 6 to Xcode 7 on my MacBook Pro running Yosemite.After opening a previously saved Swift playground, I attempted to make changes in it. More often than not, this results in the following error message and the playground does not execute.Failed to launch iOS stub for playground execution: error: attach failed: unable to attach.I tried creating a new playground and the behavior is the same. Eventually, making a change results in the error above.The only way to recover and it's only temporary because the error reoccurs is to stop and restart Xcode.