Fix alerts crash when controller is not in view hierarchy

[vladpcog]

Summary

• Check that the view controller is present in the view hierarchy before trying to present the alert, so as to avoid crashes when the view is not in the hierarchy.
• Check that there is no other alert already presented by the implementor, so as to avoid the completion handler never getting called and crashing the app.

⚠️ I removed the testTarget part from the Package.swift, because I could not build the package in Xcode 14.1, as there was no testTarget in the package.