Fix prefab close dialog Editor crash on Linux (#4623)

Signed-off-by: Steve Pham <spham@amazon.com>

Code/Framework/AzToolsFramework/AzToolsFramework/UI/Prefab/PrefabIntegrationManager.cpp

@@ -1400,7 +1400,7 @@ namespace AzToolsFramework
 
         AZStd::unique_ptr<AzQtComponents::Card> PrefabIntegrationManager::ConstructUnsavedPrefabsCard(TemplateId templateId)
         {
-            FlowLayout* unsavedPrefabsLayout = new FlowLayout(AzToolsFramework::GetActiveWindow());
+            FlowLayout* unsavedPrefabsLayout = new FlowLayout(nullptr);
 
             AZStd::set<AZ::IO::PathView> dirtyTemplatePaths = s_prefabSystemComponentInterface->GetDirtyTemplatePaths(templateId);