Fix compiler warning when downcasting ViewController

writeas/AppDelegate.swift

@@ -45,7 +45,7 @@ class AppDelegate: NSObject, NSApplicationDelegate {
     
     func applicationDidBecomeActive(_ notification: Notification) {
         if vc == nil {
-            vc = NSApplication.shared().mainWindow?.contentViewController as! ViewController
+            vc = NSApplication.shared().mainWindow?.contentViewController as? ViewController
         }
     }