Fix crash when ProfileHeaderView tries to create observers after ProfileVC is deinit'd

Can happen if the network is slow and the user closes the profile screen before the header loads

Tusker/Views/Profile Header/ProfileHeaderView.swift

@@ -86,6 +86,11 @@ class ProfileHeaderView: UIView {
     }
     
     private func createObservers() {
+        // mastodonController may be nil if the ProfileViewController is deinit'd before the header is even created
+        guard let mastodonController else {
+            return
+        }
+        
         cancellables = []
         
         mastodonController.persistentContainer.accountSubject