Add context check for __initRetinaScaling

[andrejslogins]

Related to this #7705

In some cases, the context may not be defined.
Likely due to a function being called before the canvas is initialized, or another canvas instance is created in the old one's stead

[ShaMan123]

@andrejslogins I don't like this.
You don't know if the problem is at your end or not.
You'll be patching fabric with checks probably because you didn't find the source of the problem.
Investigate and return with insights and a repro or something so we can understand as well and give feedback.
This is bad practice.

Likely due to a function being called before the canvas is initialized, or another canvas instance is created in the old one's stead

Check your cleanup methods.
Check resizing events on canvas that may trigger this behavior.
Check initializing fabric more than once and throw an error if so (likely caused from some event you are listening to).

[asturur]

yes i would like to know how do we get to upperCanvas or lowerCanvas without the context.
When we do create them they are assigned immedialtey.

With the previous case of cacheContext that was a little bit less steady, because the cache gets created on the fly and during resizing or other events you may end up destroying and recreating it.

But this is less clear on how it happens

[andrejslogins]

i'll close this for now, issue persists, but i'm not actively working on it atm

[ShaMan123]

@andrejslogins try confirming your issue persists after patching fabric with #7775