Update RelayCommand.cs

Author: yanjinhuagood

src/WPFDevelopers.Samples.Shared/Helpers/RelayCommand.cs

@@ -8,6 +8,10 @@ public class RelayCommand : ICommand
         private readonly Func<object, bool> canExecuteFunc;
         private readonly Action<object> executeAction;
 
+        public RelayCommand()
+        {
+        }
+
         public RelayCommand(Action<object> execute)
             : this(execute, null)
         {