Fix crash produced when adding a custom field on a Secure Note, because it try to load the control of linked fields even if it's not the type (#1668)

src/App/Pages/Vault/AddEditPageViewModel.cs

@@ -929,7 +929,7 @@ namespace Bit.App.Pages
 
         public List<KeyValuePair<string, LinkedIdType>> LinkedFieldOptions
         {
-            get => _cipher.LinkedFieldOptions
+            get => _cipher.LinkedFieldOptions?
                 .Select(kvp => new KeyValuePair<string, LinkedIdType>(_i18nService.T(kvp.Key), kvp.Value))
                 .ToList();
         }