fix: Fix color picker button binding failure issue

Microsoft.Toolkit.Uwp.UI.Controls.Input/ColorPicker/ColorPicker.xaml

@@ -44,6 +44,7 @@
                 <localconverters:ColorToColorShadeConverter x:Key="ColorToColorShadeConverter" />
                 <converters:ColorToDisplayNameConverter x:Key="ColorToDisplayNameConverter" />
                 <localconverters:ColorToHexConverter x:Key="ColorToHexConverter" />
+                <localconverters:ColorToSelectedValueConverter x:Key="ColorToSelectedValueConverter" />
 
                 <Style x:Key="InputTextBoxStyle"
                        TargetType="TextBox">
@@ -157,7 +158,7 @@
                               Padding="0"
                               ItemContainerStyle="{StaticResource PaletteGridViewItemStyle}"
                               ItemsSource="{TemplateBinding CustomPaletteColors}"
-                              SelectedValue="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Color, Mode=TwoWay}"
+                              SelectedValue="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Color, Mode=TwoWay, Converter={StaticResource ColorToSelectedValueConverter}}"
                               SelectionMode="Single"
                               Tag="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Color, Mode=OneWay}">
                         <GridView.ItemsPanel>

Microsoft.Toolkit.Uwp.UI.Controls.Input/ColorPicker/ColorToSelectedValueConverter.cs

@@ -0,0 +1,36 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Data;
+
+namespace Microsoft.Toolkit.Uwp.UI.Controls.ColorPickerConverters
+{
+    /// <summary>
+    /// Ignores null target values for TwoWay binding.
+    /// </summary>
+    public class ColorToSelectedValueConverter : IValueConverter
+    {
+        /// <inheritdoc/>
+        public object Convert(
+            object value,
+            Type targetType,
+            object parameter,
+            string language)
+        {
+            return value;
+        }
+
+        /// <inheritdoc/>
+        public object ConvertBack(
+            object value,
+            Type targetType,
+            object parameter,
+            string language)
+        {
+            return value ?? DependencyProperty.UnsetValue;
+        }
+    }
+}