Notices like this one help developers to identify potential problems but don't necessarily indicate that there is any actual problem. So, ordinarily, the plugin will be operationally unaffected by this (there are some exceptions such as prematurely causing headers to be dispatched or "fouling" ajax responses, though).
Best advice in the context of a production (or live) site is to turn off the display of errors. If you require WP_DEBUG to be on within a development environment - and don't want strict or notice level error reporting - you would have to either use a PHP directive to set the error reporting level appropriately such that it cannot be overriden during execution, or else set it with ini_set() after WordPress carries out its error-level setting magic, from either an early-running action or even within a mu-plugin.
As Rob noted we have talked to the dev team about this and they will indeed be driving forward a cleanup to help ensure compliance with strict coding standards - so although you may not see a change in the very next release this sort of thing will definitely be addressed in the not too distant future.