diff --git a/Packages/vChewing_CocoaExtension/Sources/CocoaExtension/CocoaExtension_NSWindowController.swift b/Packages/vChewing_CocoaExtension/Sources/CocoaExtension/CocoaExtension_NSWindowController.swift
index 262320df..7f2d38cf 100644
--- a/Packages/vChewing_CocoaExtension/Sources/CocoaExtension/CocoaExtension_NSWindowController.swift
+++ b/Packages/vChewing_CocoaExtension/Sources/CocoaExtension/CocoaExtension_NSWindowController.swift
@@ -48,6 +48,16 @@ extension NSWindowController {
   }
 }
 
+extension NSWindow {
+  public func callAlert(title: String, text: String? = nil) {
+    let alert = NSAlert()
+    alert.messageText = title
+    if let text = text { alert.informativeText = text }
+    alert.addButton(withTitle: NSLocalizedString("OK", comment: ""))
+    alert.beginSheetModal(for: self)
+  }
+}
+
 extension IMKCandidates {
   /// 設定選字窗的顯示位置。
   ///
diff --git a/Source/Modules/WindowControllers/CtlClientListMgr.swift b/Source/Modules/WindowControllers/CtlClientListMgr.swift
index 653ab150..c8ae4a97 100644
--- a/Source/Modules/WindowControllers/CtlClientListMgr.swift
+++ b/Source/Modules/WindowControllers/CtlClientListMgr.swift
@@ -43,16 +43,6 @@ extension CtlClientListMgr {
     PrefMgr.shared.clientsIMKTextInputIncapable.count
   }
 
-  func callAlert(_ window: NSWindow, title: String, text: String? = nil) {
-    let alert = NSAlert()
-    alert.messageText = title
-    if let text = text {
-      alert.informativeText = text
-    }
-    alert.addButton(withTitle: NSLocalizedString("OK", comment: ""))
-    alert.beginSheetModal(for: window)
-  }
-
   @IBAction func btnAddClientClicked(_: Any) {
     guard let window = window else { return }
     let alert = NSAlert()
@@ -119,18 +109,18 @@ extension CtlClientListMgr {
                   )
                   let text = url.path + "\n\n" + NSLocalizedString("Please try again.", comment: "")
                   guard let bundle = Bundle(url: url) else {
-                    self.callAlert(window, title: title, text: text)
+                    self.window?.callAlert(title: title, text: text)
                     return
                   }
                   guard let identifier = bundle.bundleIdentifier else {
-                    self.callAlert(window, title: title, text: text)
+                    self.window?.callAlert(title: title, text: text)
                     return
                   }
                   if PrefMgr.shared.clientsIMKTextInputIncapable.contains(identifier) {
                     title = NSLocalizedString(
                       "The selected item's identifier is already in the list.", comment: ""
                     )
-                    self.callAlert(window, title: title, text: identifier + "\n\n" + url.path)
+                    self.window?.callAlert(title: title, text: identifier + "\n\n" + url.path)
                     return
                   }
                   self.applyNewValue(identifier)