Tolerate a null file in DelayedFileSource

[FrayxRulez]

NullReferenceException — "Object reference not set to an instance of an object", reported by crash telemetry on 12.9.1.0.

0  Telegram.Streams.DelayedFileSource.get_Id
   Telegram/Streams/DelayedFileSource.cs:181
1  Telegram.Streams.DelayedFileSource.GetHashCode
   Telegram/Streams/DelayedFileSource.cs:228
2  System.Collections.Generic.ObjectEqualityComparer`1.GetHashCode
3  Telegram.Controls.AnimatedImagePresentation.GetHashCode
4  Telegram.Controls.AnimatedImageLoader.GetOrCreate
   Telegram/Controls/AnimatedImage.cs:1857
5  Telegram.Controls.AnimatedImage.Load
   Telegram/Controls/AnimatedImage.cs:519
6  Telegram.Controls.AnimatedImage.OnLoaded
   Telegram/Controls/AnimatedImage.cs:122

Cause

A DelayedFileSource built from a sticker whose StickerValue is null ends up with a null _file. That is a supported state everywhere else in the hierarchy — the constructor guards if (file != null), FilePath and IsDownloadingCompleted use _file?., LocalFileSource returns early on a null file, and all four delayed subclasses override Id because their _file starts null. The base Id was the one accessor that dereferenced it unconditionally, and GetHashCode (HashCode.Combine(Id, IsAnimated)) makes it unavoidable as soon as AnimatedImageLoader.GetOrCreate hashes the presentation. The crashing instance is the base class, not a subclass.

EmojiDrawerViewModel deliberately inserts placeholder stickers with a null StickerValue, so this is reachable from the emoji drawer.

Change

• DelayedFileSource.Id falls back to 0 on a null file, matching LocalFileSource, whose Id is left at 0 for exactly this case.
• DelayedFileSource.DownloadFile returns early on a null file instead of dereferencing it three times. Subclasses fully override this method to resolve the file themselves, so nothing in that path is lost.
• EmojiDrawer's "+N" expand path assigns a file source without the StickerValue != null check that its two sibling call sites in the same file already have; added.

Not built — a .NET Native build isn't available here. Both files parse clean under CSharpSyntaxTree.ParseText(...).GetDiagnostics(), which catches syntax errors and nothing more.

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