BaseExtEvent._persistentListeners is null unless serialized

[harald921]

Issue: The array "_persistentListeners" is null unless serialized. This causes a NullReferenceException in events that have been added during runtime specifically the use of DynamicListeners.

Steps to reproduce:

• Create a new MonoBehaviour named "Test" with a serialized ExtEvent "TestEvent"
• Put "Test" on a prefab
• Instantiate prefab
• Try and invoke TestEvent in any way (With or without DynamicListeners having any listeners)

Solution: Create a new empty array in the constructor

(Off topic side note: Thank you so much for creating this plugin, it's amazing)