#3382 Should check if _backgroundJobServer is null

If you disable job execution, it will be null!

framework/src/Volo.Abp.HangFire/Volo/Abp/Hangfire/AbpHangfireModule.cs

@@ -25,9 +25,11 @@ namespace Volo.Abp.Hangfire
 
         public override void OnApplicationShutdown(ApplicationShutdownContext context)
         {
-            //TODO: ABP may provide two methods for application shutdown: OnPreApplicationShutdown & OnApplicationShutdown
-            _backgroundJobServer.SendStop();
-            _backgroundJobServer.Dispose();
+            if (_backgroundJobServer != null)
+            {
+                _backgroundJobServer.SendStop();
+                _backgroundJobServer.Dispose();
+            }
         }
     }
 }