Temporary fix for crash that occurs when switching accounts immediately

after adding a new one

Tusker/Screens/Preferences/PreferencesNavigationController.swift

@@ -57,8 +57,16 @@ class PreferencesNavigationController: UINavigationController {
     }
     
     @objc func activateAccount(_ notification: Notification) {
+        // TODO: this is a temporary measure
+        // when switching accounts shortly after adding a new one, there is an old instance of PreferncesNavigationController still around
+        // which tries to handle the notification but is unable to because it no longer is in a window (and therefore doesn't have a scene delegate)
+        // the propper fix would be to figure out what's leaking instances of this class
+        guard let window = self.view.window,
+            let windowScene = window.windowScene,
+            let sceneDelegate = windowScene.delegate as? SceneDelegate else {
+                return
+        }
         let account = notification.userInfo!["account"] as! LocalData.UserAccountInfo
-        let sceneDelegate = self.view.window!.windowScene!.delegate as! SceneDelegate
         isSwitchingAccounts = true
         dismiss(animated: true) { // dismiss preferences
             sceneDelegate.activateAccount(account)
@@ -66,7 +74,11 @@ class PreferencesNavigationController: UINavigationController {
     }
     
     @objc func userLoggedOut() {
-        let sceneDelegate = self.view.window!.windowScene!.delegate as! SceneDelegate
+        guard let window = self.view.window,
+            let windowScene = window.windowScene,
+            let sceneDelegate = windowScene.delegate as? SceneDelegate else {
+                return
+        }
         isSwitchingAccounts = true
         dismiss(animated: true) { // dismiss preferences
             sceneDelegate.logoutCurrent()