remove plugin list caching

Removes the CordovaRegistryPluginInfo caching which provides a little
benefit 

Signed-off-by: Gorkem Ercan <gorkem.ercan@gmail.com>
Check for disposed widgets

Adds a safeguard against control being disposed, before making a UI call

Signed-off-by: Gorkem Ercan <gorkem.ercan@gmail.com>