fix: if UIParticle's parents do not have Canvas, an exception is thro…

…wn in OnEnable

close #300

Packages/src/Runtime/UIParticleRenderer.cs

@@ -514,7 +514,8 @@ private Vector3 GetWorldScale()
             var scale = _parent.scale3DForCalc.GetScaled(_parent.parentScale);
 
             if (_parent.autoScalingMode == UIParticle.AutoScalingMode.UIParticle
-                && _particleSystem.main.scalingMode == ParticleSystemScalingMode.Local)
+                && _particleSystem.main.scalingMode == ParticleSystemScalingMode.Local
+                && _parent.canvas)
             {
                 scale = scale.GetScaled(_parent.canvas.transform.localScale);
             }