修复TagIpoutBUG

修复TextBox值为Null无法删除问题以及AllowDuplicates为false状态 VM中的Tags为null时的异常状态问题

demo/Ursa.Demo/Pages/TagInputDemo.axaml

@@ -12,7 +12,9 @@
         <u:TagInput
             Name="labels"
             Margin="20"
-            Separator="-" />
+            AllowDuplicates="False"
+            Separator="-"
+            Tags="{Binding Tags}" />
         <u:TagInput
             Margin="20"
             AllowDuplicates="False"

demo/Ursa.Demo/ViewModels/TagInputDemoViewModel.cs

@@ -1,6 +1,13 @@
+using System.Collections.ObjectModel;
+
 namespace Ursa.Demo.ViewModels;
 
 public class TagInputDemoViewModel: ViewModelBase
 {
-    
+    private ObservableCollection<string> _Tags ;
+    public ObservableCollection<string> Tags
+    {
+        get { return _Tags; }
+        set { SetProperty(ref _Tags, value); }
+    }
 }

src/Ursa/Controls/TagInput/TagInput.cs

@@ -210,24 +210,22 @@ public class TagInput : TemplatedControl
                     values = new[] { _textBox.Text };
                 }
 
-                if (!AllowDuplicates)
-                {
+                if (!AllowDuplicates && Tags != null)
                     values = values.Distinct().Except(Tags).ToArray();
-                }
 
                 for (int i = 0; i < values.Length; i++)
                 {
                     int index = Items.Count - 1;
                     // Items.Insert(index, values[i]);
-                    Tags.Insert(index, values[i]);
+                    Tags?.Insert(index, values[i]);
                 }
 
                 _textBox.Text = "";
             }
         }
         else if (args.Key == Key.Delete || args.Key == Key.Back)
-        {
-            if (_textBox.Text?.Length == 0)
+        { 
+            if (string.IsNullOrEmpty(_textBox.Text)||_textBox.Text?.Length == 0)
             {
                 if (Tags.Count == 0)
                 {