Use topViewController not rootViewController

[naoyanickf]

I changed topViewController function of DialogManager.swift.

My fix use topViewController not rootViewController when topViewController is available.

In my case,

• When launching app, I check whether user is already logged in or not.
• If logged in, I present mainViewController quickly.
• Update alert didn't show up because of "whose view is not in the window hierarchy!" error.

So, I'm using the most top topViewController when it's available.

Swift 4.2 Xcode 10.2.1

[dtrenz]

Released in 2.0.0.