Exception copying string

```
Exception Info: System.NullReferenceException
at StringTheory.UI.StringListPage+<>c.<.ctor>g__CopyStrings|53_1(System.Collections.IList)
at StringTheory.UI.DelegateCommand`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Execute(System.Object)
at System.Windows.Input.CommandManager.TranslateInput(System.Windows.IInputElement, System.Windows.Input.InputEventArgs)
at System.Windows.UIElement.OnKeyDownThunk(System.Object, System.Windows.Input.KeyEventArgs)
at System.Windows.Input.KeyEventArgs.InvokeEventHandler(System.Delegate, System.Object)
at System.Windows.RoutedEventArgs.InvokeHandler(System.Delegate, System.Object)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(System.Object, System.Windows.RoutedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(System.Object, System.Windows.RoutedEventArgs, Boolean)
at System.Windows.UIElement.RaiseEventImpl(System.Windows.DependencyObject, System.Windows.RoutedEventArgs)
at System.Windows.UIElement.RaiseTrustedEvent(System.Windows.RoutedEventArgs)
at System.Windows.UIElement.RaiseEvent(System.Windows.RoutedEventArgs, Boolean)
at System.Windows.Input.InputManager.ProcessStagingArea()
at System.Windows.Input.InputManager.ProcessInput(System.Windows.Input.InputEventArgs)
at System.Windows.Input.InputProviderSite.ReportInput(System.Windows.Input.InputReport)
at System.Windows.Interop.HwndKeyboardInputProvider.ReportInput(IntPtr, System.Windows.Input.InputMode, Int32, System.Windows.Input.RawKeyboardActions, Int32, Boolean, Boolean, Int32)
at System.Windows.Interop.HwndKeyboardInputProvider.ProcessKeyAction(System.Windows.Interop.MSG ByRef, Boolean ByRef)
at System.Windows.Interop.HwndSource.CriticalTranslateAccelerator(System.Windows.Interop.MSG ByRef, System.Windows.Input.ModifierKeys)
at System.Windows.Interop.HwndSource.OnPreprocessMessage(System.Object)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority, System.TimeSpan, System.Delegate, System.Object, Int32)
at System.Windows.Threading.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority, System.Delegate, System.Object)
at System.Windows.Interop.HwndSource.OnPreprocessMessageThunk(System.Windows.Interop.MSG ByRef, Boolean ByRef)
at System.Windows.Interop.HwndSource+WeakEventPreprocessMessage.OnPreprocessMessage(System.Windows.Interop.MSG ByRef, Boolean ByRef)
at System.Windows.Interop.ComponentDispatcherThread.RaiseThreadMessage(System.Windows.Interop.MSG ByRef)
at System.Windows.Threading.Dispatcher.PushFrameImpl(System.Windows.Threading.DispatcherFrame)
at System.Windows.Threading.Dispatcher.PushFrame(System.Windows.Threading.DispatcherFrame)
at System.Windows.Application.RunDispatcher(System.Object)
at System.Windows.Application.RunInternal(System.Windows.Window)
at System.Windows.Application.Run(System.Windows.Window)
at StringTheory.UI.App.Main()
```

Start at StringTheory.UI.StringListPage and inspect the CopyStrings entry point named in the exception, then reproduce the copy action that triggers the NullReferenceException. Trace which value is null and check any nearby tests or existing handling before defining the expected behavior; done means the action no longer throws for the reported case.

Written by the indexing model from the issue text.