DownloadLinkModel doesn't check if it actually receives a valid asset from DAMTemplatingFuncitons. If the asset doesn't exist, it wraps null inside a LegacyAssetWrapper, therefore the template produces an exception while accessing methods on the legacy wrapper.

Possible fix in DownloadLinkModel (check asset before returning wrapper):

    public Asset getAsset() {
        final String assetIdentifier = PropertyUtil.getString(this.content, "link");
        if (StringUtils.isNotBlank(assetIdentifier) && null != this.damTemplatingFunctions.getAsset(assetIdentifier)) {
            return new LegacyAssetWrapper(this.damTemplatingFunctions.getAsset(assetIdentifier));
        }
        return null;
    }

The template will then show the "link does not exists" error message as before (in Magnolia 4.5).

Related to MGNLSTK-1434 which would also be fixed with this.

Cheers
Andreas