ControlUtils: Fix FinAllChildren

Elwig/App.xaml.cs

@@ -157,9 +157,12 @@ namespace Elwig {
 
         protected void OnPrintingReadyChanged(EventArgs evt) {
             foreach (Window w in Windows) {
-                foreach (var b in ControlUtils.FindAllChildren<Button>(w).Where(b => "Print".Equals(b.Tag))) {
+                foreach (var b in ControlUtils.FindAllChildren<Button>(w).Where(b => b.Tag?.ToString() == "Print")) {
                     b.IsEnabled = IsPrintingReady;
                 }
+                foreach (var i in ControlUtils.FindAllChildren<MenuItem>(w).Where(i => i.Tag?.ToString() == "Print")) {
+                    i.IsEnabled = IsPrintingReady;
+                }
             }
         }
     }