Fix devtools not making screenshots

src/Avalonia.Diagnostics/Diagnostics/Screenshots/FilePickerHandler.cs

@@ -1,4 +1,5 @@
-using System.IO;
+using System;
+using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
 using Avalonia.Controls;
@@ -57,31 +58,25 @@ namespace Avalonia.Diagnostics.Screenshots
             return window!;
         }
 
-        protected async override Task<Stream?> GetStream(IControl control)
+        protected override async Task<Stream?> GetStream(IControl control)
         {
-            Stream? output = default;
             var result = await GetWindow(control).StorageProvider.SaveFilePickerAsync(new FilePickerSaveOptions
             {
                 SuggestedStartLocation = new BclStorageFolder(new DirectoryInfo(ScreenshotsRoot)),
                 Title = Title,
-                FileTypeChoices = new FilePickerFileType[] { new FilePickerFileType("PNG") { Patterns = new string[] { "*.png" } } }
+                FileTypeChoices = new FilePickerFileType[] { new FilePickerFileType("PNG") { Patterns = new string[] { "*.png" } } },
+                DefaultExtension = ".png"
             });
-
-            if (result!=null && !string.IsNullOrWhiteSpace(result.Name))
+            if (result is null)
+            {
+                return null;
+            }
+            if (!result.CanOpenWrite)
             {
-                var folder = Path.GetDirectoryName(result.Name);
-                // Directory information for path, or null if path denotes a root directory or is
-                // null. Returns System.String.Empty if path does not contain directory information.
-                if (!string.IsNullOrWhiteSpace(folder))
-                {
-                    if (!Directory.Exists(folder))
-                    {
-                        Directory.CreateDirectory(folder);
-                    }
-                    output = new FileStream(result.Name, FileMode.Create);
-                }
+                throw new InvalidOperationException("ReadOnly file was opened");
             }
-            return output;
+
+            return await result.OpenWriteAsync();
         }
     }
 }