Resolved the command error.

src/Avalonia.Dialogs/AboutAvaloniaDialog.xaml

@@ -44,7 +44,6 @@
             
       <TextBlock Text="v11"
                  FontWeight="Regular"
-                 Foreground="#5B5B5B"
                  HorizontalAlignment="Center"
                  FontSize="14"
                  Grid.Row="2"/>
@@ -57,9 +56,8 @@
                  TextWrapping="Wrap"/>
         
       <TextBlock Grid.Row="1" 
-                 Text="Avalonia is made possible by the generous support if it's contributors and community"
+                 Text="Avalonia is made possible by the generous support if it's contributors and community."
                  TextWrapping="Wrap"
-                 Foreground="#5B5B5B"
                  Margin="0,15,0,0"/>
         
       <Button Grid.Row="2" Background="#8B44AC"
@@ -67,12 +65,11 @@
               VerticalAlignment="Center"
               Padding="12"
               Content="Learn more about Avalonia"
-              Command="{Binding LearnMore}"
+              Click="Button_OnClick"
               HorizontalAlignment="Center"/>
             
       <TextBlock Grid.Row="4" VerticalAlignment="Bottom"
                  Text="{Binding Copyright}"
-                 Foreground="#5B5B5B"
                  HorizontalAlignment="Center"/>
     </Grid>
   </Grid>

src/Avalonia.Dialogs/AboutAvaloniaDialog.xaml.cs

@@ -3,6 +3,7 @@ using System.Diagnostics;
 using System.Runtime.InteropServices;
 using System.Text.RegularExpressions;
 using Avalonia.Controls;
+using Avalonia.Interactivity;
 using Avalonia.Markup.Xaml;
 
 namespace Avalonia.Dialogs
@@ -23,26 +24,7 @@ namespace Avalonia.Dialogs
             DataContext = this;
         }
 
-        public static void LearnMore()
-        {
-            var url = "https://www.avaloniaui.net/";
-            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
-            {
-                // If no associated application/json MimeType is found xdg-open opens retrun error
-                // but it tries to open it anyway using the console editor (nano, vim, other..)
-                ShellExec($"xdg-open {url}", waitForExit: false);
-            }
-            else
-            {
-                using Process process = Process.Start(new ProcessStartInfo
-                {
-                    FileName = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? url : "open",
-                    Arguments = RuntimeInformation.IsOSPlatform(OSPlatform.OSX) ? $"{url}" : "",
-                    CreateNoWindow = true,
-                    UseShellExecute = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
-                });
-            }
-        }
+        
 
         private static void ShellExec(string cmd, bool waitForExit = true)
         {
@@ -67,5 +49,26 @@ namespace Avalonia.Dialogs
                 }
             }
         }
+
+        private void Button_OnClick(object sender, RoutedEventArgs e)
+        {
+            var url = "https://www.avaloniaui.net/";
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
+            {
+                // If no associated application/json MimeType is found xdg-open opens retrun error
+                // but it tries to open it anyway using the console editor (nano, vim, other..)
+                ShellExec($"xdg-open {url}", waitForExit: false);
+            }
+            else
+            {
+                using Process process = Process.Start(new ProcessStartInfo
+                {
+                    FileName = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? url : "open",
+                    Arguments = RuntimeInformation.IsOSPlatform(OSPlatform.OSX) ? $"{url}" : "",
+                    CreateNoWindow = true,
+                    UseShellExecute = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
+                });
+            }
+        }
     }
 }