fix: NullReferenceException in SplitButton

[Nuklon]

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

Under some cases, a NullReferenceException can be thrown by SplitButton:

Description: The process was terminated due to an unhandled exception.
Exception Info: System.NullReferenceException: Object reference not set to an instance of an object.
   at Wpf.Ui.Controls.SplitButton.ReleaseTemplateResources()
   at Wpf.Ui.Controls.SplitButton.<>c.<.ctor>b__14_0(Object sender, RoutedEventArgs _)

What is the new behavior?

Checks if type is non-null.