Fix NRE on Application Exit.

Fixing an NRE that can happen if attempting to start tween on application exit

Author: Cory Leach

Runtime/Tween/TweenExtensions.cs

@@ -11,7 +11,7 @@ public static class TweenExtensions
         private static CancellationTokenSource _cancellationTokenSource;
         private static Dictionary<int, TweenData> _tweenDict;
 
-        private static CancellationToken CancellationToken => _cancellationTokenSource.Token;
+        private static CancellationToken CancellationToken => (_cancellationTokenSource != null) ? _cancellationTokenSource.Token : CancellationToken.None;
 
         private static bool CanTween => Application.isPlaying && !CancellationToken.IsCancellationRequested;
 
@@ -81,10 +81,16 @@ public static Task DoPunchTweenAsync(int id, float duration, CancellationToken c
 
         public static async Task DoTweenAsyncWithLerp(Func<float,float,float,float> lerpMethod, int id, float duration, CancellationToken cancellationToken, Action<float> action, Easing easeType = Easing.Linear, AnimationCurve customCurve = null)
         {
+            if (!CanTween)
+            {
+                return;
+            }
+
             var instanceCancellationToken = StartTween(id);
 
             float t = 0;
             var ease = easeType != Easing.CustomCurve ? EaseFunctions.Get(easeType) : customCurve.Evaluate;
+
             action?.Invoke(ease.Invoke(0));
 
             while (t < duration && CanTween)