fix FlyoutsControl memory leak and exception

[punker76]

Closes #1638 Update FlyoutsControl.cs

i'm using a <Controls:FlyoutsControl ItemsSource="{Binding Flyouts}" /> binded to an >ObservableCollection but when i remove items from Flyout Collection i get a Value can't be null >error.

Following the recent conversation with thoemmi in Gitter i changed FlyoutControl to check for null before >advancing

this pr uses PropertyChangeNotifier from this nice solution

AddValueChanged of dependency property descriptor results in memory leak as you already know.
So, as described here, you can create custom class PropertyChangeNotifier to listen
to any dependency property changes.

This class takes advantage of the fact that bindings use weak references to manage associations
so the class will not root the object who property changes it is watching. It also uses a WeakReference
to maintain a reference to the object whose property it is watching without rooting that object.
In this way, you can maintain a collection of these objects so that you can unhook the property
change later without worrying about that collection rooting the object whose values you are watching.

[flagbug]

Should this be internal?

[punker76]

maybe, it's not only for mahapps...