mainInstance() Can Crash the Application

[FutureSeth]

We found ourselves in a state where MixPanel was being torn down before being setup Mixpanel.mainInstance.reset() and it was crashing our production application through an assert. It's understandable that there's an order of operations expected when using your SDK but this should be handled gracefully or provide a way to check if it's safe to call mainInstance.

    open class func mainInstance() -> MixpanelInstance {
        if let instance = MixpanelManager.sharedInstance.getMainInstance() {
            return instance
        } else {
            #if !targetEnvironment(simulator)
            assert(false, "You have to call initialize(token:trackAutomaticEvents:) before calling the main instance, " +
                   "or define a new main instance if removing the main one")
            #endif
            
#if !os(OSX) && !os(watchOS)
            return Mixpanel.initialize(token: "", trackAutomaticEvents: true)
#else
            return Mixpanel.initialize(token: "")
#endif
            
        }
    }

[jaredmixpanel]

@FutureSeth How about this? #668

[jaredmixpanel]

#668 is released with v4.4.0