Retrieving Stream field throws null reference when GetBlobStream returns null (e.g. when media file is detached)

[mikemuggli]

Using Glass Mapper 5.6.160 on Sitecore 9.3.

In a scenario where a field is mapped to a Stream type, for example the Blob field of the media File type (/sitecore/templates/System/Media/Unversioned/File/Media/Blob) if the Blob field is empty/null (e.g. the media asset was detached), Glass Mapper is throwing a null reference exception.

I believe this happens in the GetField method of the SitecoreFieldStreamMapper class. It tries to get the blob stream:

var data = field.GetBlobStream();

In the case of a detached asset, data will be null. But there is no null check before it tries to use data in the conditional:

if (data.CanRead) - https://github.com/mikeedwards83/Glass.Mapper/blob/master/Source/Glass.Mapper.Sc/DataMappers/SitecoreFieldStreamMapper.cs#:~:text=if%20(-,data.CanRead,-)

I believe the above line is where the null reference exception occurs.