Failure to translate a block that contains block properties

Description

Steps to reproduce

1. Create a block type that contains 2 block properties.
2. Create a block in English then publish.
3. Create a draft version in Swedish.
4. Add the draft version of the block to a project. Then, follow the instructions to export and import that block for translating (Importing and exporting XLIFF).

http://webhelp.episerver.com/latest/addons/languages.htm

Expected: The project is successfully translated.

Actual: Failure to translate the project with the following exception:

2018-05-10 10:00:20,288 [80] ERROR EPiServer.Labs.LanguageManager.Business.XLIFFTranslationPackageImporter: Exception when import translation package
System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
   at System.ThrowHelper.ThrowKeyNotFoundException()
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at EPiServer.Labs.LanguageManager.Business.XLIFFTranslationPackageImporter.ImportTranslationFromXmlString(String xmlString, String fileName, Int32 projectId)
   at EPiServer.Labs.LanguageManager.Business.XLIFFTranslationPackageImporter.ImportTranslationPackage(String packageName, Int32& projectId, String& message)
System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
   at System.ThrowHelper.ThrowKeyNotFoundException()
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at EPiServer.Labs.LanguageManager.Business.XLIFFTranslationPackageImporter.ImportTranslationFromXmlString(String xmlString, String fileName, Int32 projectId)
   at EPiServer.Labs.LanguageManager.Business.XLIFFTranslationPackageImporter.ImportTranslationPackage(String packageName, Int32& projectId, String& message)