Fix crash showing custom instance on iOS 14

Tusker/Screens/Onboarding/InstanceSelectorTableViewController.swift

@@ -118,7 +118,9 @@ class InstanceSelectorTableViewController: UITableViewController {
         let request = Client.getInstance()
         client.run(request) { (response) in
             var snapshot = self.dataSource.snapshot()
+            if snapshot.indexOfSection(.selected) != nil {
                 snapshot.deleteItems(snapshot.itemIdentifiers(inSection: .selected))
+            }
 
             if case let .success(instance, _) = response {
                 if !snapshot.sectionIdentifiers.contains(.selected) {