ActionCommand: Small fixes to avoid warnings

Elwig/Helpers/ActionCommand.cs

@@ -4,18 +4,18 @@ using System.Windows.Input;
 namespace Elwig.Helpers {
     public class ActionCommand : ICommand {
 
-        public event EventHandler CanExecuteChanged;
+        public event EventHandler? CanExecuteChanged;
         private readonly Action Action;
 
         public ActionCommand(Action action) {
             Action = action;
         }
 
-        public void Execute(object parameter) {
+        public void Execute(object? parameter) {
             Action();
         }
 
-        public bool CanExecute(object parameter) {
+        public bool CanExecute(object? parameter) {
             return true;
         }
     }