ScenarioManager: don't try to set non-existent IsInitializing property

* it exists only in vse branch where it was added with this commit:
  https://auto-gitlab.lgsvl.net/HDRP/Simulator/commit/08038b7a2b87e1608aebe757e74d6acd774006fb#b8a2054485aa4aa693a454077a9753bb5fed45cc
  but it doesn't exist in master causing:
  04:10:58  Assets/Scripts/ScenarioEditor/Managers/ScenarioManager.cs(310,13): error CS0103: The name 'IsInitializing' does not exist in the current context

* and assign to private isInitialized property directly to fix:
  04:10:58  Assets/Scripts/ScenarioEditor/Managers/ScenarioManager.cs(311,13): error CS0200: Property or indexer 'ScenarioManager.IsInitialized' cannot be assigned to -- it is read only

Assets/Scripts/ScenarioEditor/Managers/ScenarioManager.cs

@@ -307,8 +307,7 @@ private async Task Initialize()
 
         private void StopInitialization()
         {
-            IsInitializing = false;
-            IsInitialized = true;
+            isInitialized = true;
             Deinitialize();
             Loader.ExitScenarioEditor();
         }
@@ -459,4 +458,4 @@ public void NewElementActivated(ScenarioElement scenarioElement)
             NewScenarioElement?.Invoke(scenarioElement);
         }
     }
-}
+}