Prevent a crash when calling AnimationMixer::restore with an invalid resource

[jsjtxietian]

Fixes #85340

[AThousandShips]

Is this actually likely to happen when the engine calls this method? This is not a part of the exposed API and users providing invalid input is not necessarily a priority, though the fix is cheap (this method can't even really be called from GDScript as the type isn't exposed or functional from that side)

[jsjtxietian]

This is not a part of the exposed API and users providing invalid input is not necessarily a priority, though the fix is cheap

I believe it's not that high priority.

[YuriSizov]

A note for a better commit message: describe what problem is addressed rather than how you address it. The how is already obvious from the change. So the message could be "Prevent a crash when calling AnimationMixer::restore when an invalid reference".

[qarmin]

@AThousandShips it is exposed to gdscript - just look at the issue.

I remember that once there was a discussion about the fact that from GDScript you can call private api and the conclusion was that is not possible to fix the problem(at that moment), so nothing was done about it(there is definitely an issue about it somewhere)

[AThousandShips]

It isn't part of the exposed API, there's a difference, a significant one

The method can't be called properly ever as it's not a class that's available to GDScript so all this solves is a crash when using this method incorrectly

Worth it but not a part of the exposed API

Resolved

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.2.1.