【兼容】Window nil处理

PDF Office/PDF Master/Class/KMLightMember/Controller/ComparativeTable/KMComparativeTableViewController.swift

@@ -106,7 +106,9 @@ var comparativeMainWindow: NSWindow?
             controller.subscribeAction(controller: controller, isRestore:true)
         }
         
-        window.beginSheet(controller.window!)
+        if controller.window != nil {
+            window.beginSheet(controller.window!)
+        }
 
         comparativeController = controller
         comparativeMainWindow = window