Check if customEvents is null before accessing

[RobbyZ]

Thanks for this great resource :)

I found a small bug. I am checking events first before adding them to ensure I don't double-add a custom event. But if I do this, I'll get a null reference exception because customEvents is null until one is defined.

https://github.com/Elendow/SpritedowAnimator/blob/d3cb7a4729db5dd03e18abb093c13a8f6becf8b8/Source/Assets/SpritedowAnimator/Source/BaseAnimator.cs#L686

I added a null check to this if:

if (customEvents != null && customEvents.ContainsKey(eventInfo))

But maybe would be better to just always define customEvents on init? If not, there may be some other places where customEvents is accessed without null check.