DefaultMediaUrlProvider throws error when using FileSystemProvider.Azure

In an Umbraco 8.0.2 Website we use the FileSystemProvider.Azure package to load media from azure storage, but since upgrading to Umbraco 8.1 all old media files stopped working. Trying to resolve image.url produces the following exception in our api:

This happens in our mapper function:

public IDtoBase Map(IPublishedElement publishedElement, string culture = null)
        {
            if (publishedElement is Image image)
            {
                return new ImageDto
                {
                    Type = Type,
                    Id = image.Key,
                    Path = image.Url(culture, UrlMode.Absolute),
                    WidthInPixels = image.Value(prop => prop.UmbracoWidth, culture),
                    HeightInPixels = image.Value(prop => prop.UmbracoHeight, culture)
                };
            }
            return null;
        }

When using the property variant of Image.Url we get the following message:

Reproduction

• Create an Umbraco 8.0.2 website
• Add FileSystemProvider.Azure
• Upload image(s)
• Upgrade to Umbraco 8.1
• Image.Url() won’t work anymore

[rasmusjp]

If the stored URL is already absolute we should just return that value

PR: https://github.com/umbraco/Umbraco-CMS/pull/6016

[nul800sebastiaan]

PR #6016 should fix that, I’ve cherrypicked it into the 8.1 branch for release with 8.1.2.