Null guard in `ShellFlyoutTemplatedContentRenderer.HeaderContainer.UpdateMinimumHeight` (#16573)

* Null guard

* Update ShellFlyoutTemplatedContentRenderer.cs

* Update ShellFlyoutTemplatedContentRenderer.cs

---------

Co-authored-by: Shane Neuville <shane94@hotmail.com>

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellFlyoutTemplatedContentRenderer.cs

@@ -779,7 +779,7 @@ namespace Microsoft.Maui.Controls.Platform.Compatibility
 					frameLayoutView.SetMinimumHeight(minHeight);
 				}
 
-				if (PlatformView.MinimumHeight != minHeight)
+				if (PlatformView is not null && PlatformView.MinimumHeight != minHeight)
 				{
 					PlatformView.SetMinimumHeight(minHeight);
 				}