Change property LastState on TryGetLastState method and add HasState property in UndoRedoHelper

src/Avalonia.Controls/TextBox.cs

@@ -148,7 +148,8 @@ namespace Avalonia.Controls
             {
                 value = CoerceCaretIndex(value);
                 SetAndRaise(CaretIndexProperty, ref _caretIndex, value);
-                if (_undoRedoHelper.IsLastState && _undoRedoHelper.LastState?.Text == Text)
+                UndoRedoState state;
+                if (_undoRedoHelper.TryGetLastState(out state) && state.Text == Text)
                     _undoRedoHelper.UpdateLastState();
             }
         }

src/Avalonia.Controls/Utils/UndoRedoHelper.cs

@@ -42,6 +42,17 @@ namespace Avalonia.Controls.Utils
 
         public bool IsLastState => _currentNode != null && _currentNode.Next == null;
 
+        public bool TryGetLastState(out TState _state)
+        {
+            _state = default(TState);
+            if (!IsLastState)
+                return false;
+
+            _state = _currentNode.Value;
+            return true;
+        }
+
+        public bool HasState => _currentNode != null;
         public void UpdateLastState(TState state)
         {
             _states.Last.Value = state;
@@ -52,8 +63,6 @@ namespace Avalonia.Controls.Utils
             _states.Last.Value = _host.UndoRedoState;
         }
 
-        public TState? LastState => _currentNode?.Value;
-
         public void DiscardRedo()
         {
             while (_currentNode?.Next != null)