NullReferenceException when trying to load an animated PNG

[TheShihan]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

ImageSharp version

3.1.2

Other ImageSharp packages and versions

SixLabors.ImageSharp.Drawing 2.1.1

Environment (Operating system, version and so on)

Windows 11 23H2

.NET Framework version

.NET 7.0

Description

I am trying to load an animated PNG (that I previously created successfully using ImageSharp). Unfortunately I get a NullReferenceException when loading the stream:

var img = Image.Load(inputStream);

   at SixLabors.ImageSharp.Formats.Png.PngDecoderCore.Identify(BufferedReadStream stream, CancellationToken cancellationToken)
   at SixLabors.ImageSharp.Formats.ImageDecoderUtilities.Identify(IImageDecoderInternals decoder, Configuration configuration, Stream stream, CancellationToken cancellationToken)
   at SixLabors.ImageSharp.Formats.Png.PngDecoder.Decode(PngDecoderOptions options, Stream stream, CancellationToken cancellationToken)
   at SixLabors.ImageSharp.Formats.SpecializedImageDecoder`1.Decode(DecoderOptions options, Stream stream, CancellationToken cancellationToken)
   at SixLabors.ImageSharp.Formats.ImageDecoder.<>c__DisplayClass1_0.<Decode>b__0(Stream s)
   at SixLabors.ImageSharp.Formats.ImageDecoder.<WithSeekableStream>g__PeformActionAndResetPosition|11_0[T](Stream s, Int64 position, <>c__DisplayClass11_0`1& )
   at SixLabors.ImageSharp.Formats.ImageDecoder.WithSeekableStream[T](DecoderOptions options, Stream stream, Func`2 action)
   at SixLabors.ImageSharp.Formats.ImageDecoder.Decode(DecoderOptions options, Stream stream)
   at SixLabors.ImageSharp.Image.Decode(DecoderOptions options, Stream stream)
   at SixLabors.ImageSharp.Image.<>c__DisplayClass80_0.<Load>b__0(Stream s)
   at SixLabors.ImageSharp.Image.WithSeekableStream[T](DecoderOptions options, Stream stream, Func`2 action)
   at SixLabors.ImageSharp.Image.Load(DecoderOptions options, Stream stream)
   at SixLabors.ImageSharp.Image.Load(Stream stream)

The image loads fine for example in Chrome and the animation plays.

Steps to Reproduce

1. Use the image provided
2. Create a stream using (var fileStream = await file.OpenAsync(FileAccessMode.Read))
3. Try to load it with: var img = Image.Load(inputStream);
4. NullReferenceException occurs

Images

[JimBobSquarePants]

Fixed with #2674