Fix if no EventSystem present in scene

Assets/Fungus/Narrative/Scripts/DialogInput.cs

@@ -82,6 +82,11 @@ namespace Fungus
 
 		protected virtual void Update()
 		{
+            if (EventSystem.current == null)
+            {
+                return;
+            }
+
             if (currentStandaloneInputModule == null)
             {
                 if (EventSystem.current == null)
@@ -155,4 +160,4 @@ namespace Fungus
 	}
 
 }
-	
+