Merge pull request #2668 from mark-monteiro/fix-application-host-dispose

Fix ApplicationHost Dispose() method

(cherry picked from commit 622106467e)
Signed-off-by: Joshua M. Boniface <joshua@boniface.me>

Emby.Server.Implementations/ApplicationHost.cs

@@ -1801,7 +1801,7 @@ namespace Emby.Server.Implementations
                 }
 
                 _userRepository?.Dispose();
-                _displayPreferencesRepository.Dispose();
+                _displayPreferencesRepository?.Dispose();
             }
 
             _userRepository = null;