Merge pull request #6638 from amwx/FlyoutOpeningFix

FlyoutBase create presenter before calling Opening event

src/Avalonia.Controls/Flyouts/FlyoutBase.cs

@@ -215,11 +215,6 @@ namespace Avalonia.Controls.Primitives
                 }
             }
 
-            if (CancelOpening())
-            {
-                return false;
-            }
-
             if (Popup.Parent != null && Popup.Parent != placementTarget)
             {
                 ((ISetLogicalParent)Popup).SetParent(null);
@@ -236,6 +231,11 @@ namespace Avalonia.Controls.Primitives
                 Popup.Child = CreatePresenter();
             }
 
+            if (CancelOpening())
+            {
+                return false;
+            }
+
             PositionPopup(showAtPointer);
             IsOpen = Popup.IsOpen = true;
             OnOpened();