The properties panel has some of its own context members that only work in the property panel space. The "context" attribute of the space is shown in the icons across the top which change for what type of object is active, eg a camera icon for the data part of a camera object, in which case context.camera resolves to the data part of the active object, similarly for context.lamp and context.speaker for those types of object.  

This is something to look out for when refactoring code from space_properties to, for instance, the 3D toolshelf. Anyhoo a good workaround here is 

    cam = context.scene.camera.data

the "active camera", ie the one that renders the scene, or for the context object

    cam = context.object.data

and in the poll method make sure 'context.scene` and or context.scene.camera are not None, and if using the latter that context.object is not None and context.object.type (or context.active_object.type) is 'CAMERA'