Fix account add crash

[winniequinn]

As reported in #3606, it was the case that attempting to add an account could result in a crash. This fixes the problem by retaining the NSHostingController that holds the AddAccountsView view in a manner similar to how 48138b1 retained accountsAddLocalWindowController.

[winniequinn]

NOTE: This is a resubmission of #3670 which was merged to main by @vincode-io on September 18th, 2022. Unfortunately, the previously merged commit is no longer in main. (Perhaps someone clobbered it with a force push?) I verified that this fix is still necessary and appropriate before resubmitting. Hopefully this will be included in the next update!