Fix crash when opening the gallery

[FrayxRulez]

Crash

NullReferenceException: Object reference not set to an instance of an object.

Reported by crash telemetry on 12.9.0.0.

0  Telegram.Controls.Gallery.GalleryWindow.ConvertOf
   Telegram/Controls/Gallery/GalleryWindow.xaml.cs:702
1  GalleryWindow_obj1_Bindings.Invoke_M_ConvertOf_413110846
2  GalleryWindow_obj1_Bindings.CompleteUpdate
3  GalleryWindow_obj1_Bindings.Update
4  Telegram.Controls.Gallery.GalleryWindow.Load
   Telegram/Controls/Gallery/GalleryWindow.xaml.cs:793
5  Telegram.Controls.Gallery.GalleryWindow.ShowAsyncInternal
   Telegram/Controls/Gallery/GalleryWindow.xaml.cs:442

Cause

ConvertOf is a function binding:

<TextBlock Text="{x:Bind ConvertOf(ViewModel.SelectedItem, ViewModel.Position, ViewModel.TotalItems), Mode=OneWay}" />

and it dereferences its first argument straight away:

private string ConvertOf(GalleryMedia item, int index, int count)
{
    if (item.IsPersonal)

Load calls Bindings.Update(), which evaluates every binding once, including this one —
before ViewModel.SelectedItem has been assigned. The stack shows exactly that path, so
the first evaluation on open runs with a null item.

ConvertCaption, a few lines below in the same file, already guards its input
(string.IsNullOrEmpty(text?.Text)); this one was just missed.

Fix

Return an empty string when there is no item yet. The binding re-evaluates as soon as
SelectedItem is set, so the label fills in normally.

Verification

Not built or run — a UWP/.NET Native build is not available in the environment this was
prepared in. The edited file was checked with Roslyn (CSharpSyntaxTree.ParseText) and
parses with no syntax errors; that confirms syntax only, not type checking.

🤖 Generated with Claude Code