I think you have things backward on this one, as Extensions are only available to the Ajax Client and Desktop Client via WebView mode, so it should not be working in Desktop if WebView is NOT enabled. The only thing of which I can think in addition to what Anil posted, would be to ensure you have the object loaded properly such that the client has access to the underlying Extension code etc.
Here is a link to the Help for Extensions, which may be of some further use such that you can use that to confirm configuration of things.
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