Adding existence check for this.canvas on object stacking mixins

[paulconlin]

Looks to resolve Cannot read property 'bringToFront' of undefined #6065

I was having instances where the canvas instance was null and bringToFront was being called. I added this existence check to all of the object stacking mixins and matches the approach to check for this.group in the if block above.

[asturur]

you forgot all the if after the else keyword. Indeed is not building.

[asturur]

you can also have this.canvas && this.canvas.bringTo... if you want.
Is the same for me, the difference in bytes saved is minimal

[paulconlin]

🤦‍♂️sorry about that, updating now

[paulconlin]

Opted to stay with the else block, but can update to this.canvas && if you'd like

[asturur]

is fine as it is!