Impossible to import a single project when it does not contain value for the related Greenhopper customfield

Here are the steps to reproduce the issue:

1. Install a new JIRA 5.2.5 + GH 6.1.2.
2. Create a new project and one single issue.
3. Generate an XML backup.
4. Delete the project and import the project from the XML backup generated. And there will be the error message:
   

As workaround solution, it needs to uninstall the Greenhopper plugin before import the project.