ArcMap.Application is null when Addin is loaded

Hello,

I've a problem in the installation of an Add-in that we've developed for our client. The Add-in does not work correctly because the ArcMap.Application value is null when loaded. The type of this Add-in is an Extension.

The Add-In is developed with ArcObjects in the version of 10.2.1. We don't have any problem when it is tested on our environment with an ArcMap 10.2.1. Our client has a version of 10.2.0.

Does any one has an idea of this error ?

Thank you very much.

Siqin