fix: MLS account not being deleted when logging out - WPB-15774 (#2572)

Author: jullianm

wire-ios-sync-engine/Source/SessionManager/SessionManager.swift

@@ -889,17 +889,15 @@ public final class SessionManager: NSObject, SessionManagerType {
 
         requireInternal(activeUserSession.userId == account.userIdentifier, "User session and account are different")
 
-        delegate?.sessionManagerWillLogout(error: error, userSessionCanBeTornDown: { [dispatchGroup] in
-            dispatchGroup.enter()
-
+        delegate?.sessionManagerWillLogout(error: error) { [weak self] in
             activeUserSession.close(deleteCookie: deleteCookie) {
                 if deleteAccount {
-                    self.deleteAccountData(for: account)
+                    self?.deleteAccountData(for: account)
                 }
-                dispatchGroup.leave()
             }
-            self.activeUserSession = nil
-        })
+
+            self?.activeUserSession = nil
+        }
     }
 
     /// Loads a session for a given account

wire-ios-sync-engine/Source/UserSession/ZMUserSession/ZMUserSession+Authentication.swift

@@ -77,10 +77,11 @@ extension ZMUserSession {
             deleteUserKeychainItems()
         }
 
-        syncManagedObjectContext.dispatchGroup?.notify(on: .main) {
+        syncManagedObjectContext.perform {
             self.tearDown()
-            completion()
         }
+
+        completion()
     }
 
     public func logout(credentials: UserEmailCredentials, _ completion: @escaping (Result<Void, Error>) -> Void) {