Fix modern slider crash for Floating Window.

https://console.firebase.google.com/u/0/project/fiery-heat-5237/crashlytics/app/ios:com.abhishek.Clocker/issues/3db4f1d96ccc7068cf1299faf3481a81?time=last-seven-days&types=crash&sessionEventKey=cda636d65f4d44768d24b4a23093436d_1938420676542150209

Clocker/Preferences/Appearance/AppearanceViewController.swift

@@ -300,7 +300,9 @@ class AppearanceViewController: ParentViewController {
                 if DataStore.shared().shouldDisplay(ViewType.showAppInForeground) {
                     let floatingWindow = FloatingWindowController.shared()
                     floatingWindow.updateTableContent()
-                    floatingWindow.modernSlider.setNeedsDisplay(floatingWindow.modernSlider.bounds)
+                    if let slider = floatingWindow.modernSlider {
+                        slider.setNeedsDisplay(floatingWindow.modernSlider.bounds)
+                    }
 
                     if !panel {
                         floatingWindow.updatePanelColor()