Update ParentPanelController.swift

Clocker/Panel/ParentPanelController.swift

@@ -143,10 +143,12 @@ class ParentPanelController: NSWindowController {
         futureSliderRangeObserver = UserDefaults.standard.observe(\.sliderDayRange, options: [.new]) { _, change in
             if change.newValue != nil {
                 self.adjustFutureSliderBasedOnPreferences()
+                if self.modernSlider != nil {
                     self.modernSlider.reloadData()
                 }
             }
         }
+    }
 
     override func awakeFromNib() {
         super.awakeFromNib()