     }
 
     func outlineView(_ outlineView: NSOutlineView, isItemExpandable item: Any) -> Bool {
-        if let itemDict = item as? [String:Any],
-           let itemNotExpandableNumber = itemDict["isNotExpandable"] as? NSNumber {
-            return !itemNotExpandableNumber.boolValue
+        if let itemDict = item as? [String:Any] {
+            let itemNotExpandableNumber = itemDict["isNotExpandable"] as? NSNumber
+            let notExpandable = (itemNotExpandableNumber?.boolValue) ?? false
+            return !notExpandable
         }
         else {
             return false


import Cocoa

class SMMMonitorWindowController: NSWindowController, NSUserInterfaceValidations {

    @objc init() {
        super.init(window: nil)


}

extension SMMMonitorWindowController: NSUserInterfaceValidations {

    // MARK: General UI

