I think you might have found a bug, there was a similar issue in APEX 4.1, Patrick Wolf suggested not to use the apex stop engine in APEX on demand processes, but I suspect, only suspect.. :-) that the APEX team might have done it anyway.
You can more information about this here:
It might be usefull if you can reproduce this on apex.oracle.com and notify the APEX dev team about this.
Thanks for reporting this error. As Joni has suggested, it is indeed a bug, which I've just logged (17540909). There's an unrequired call to stop the APEX engine, which is resulting in the misleading error message you see reported in the debug output. We'll ensure this issue is addressed in our next major release. In the meantime, as you've seen, your chart is displaying without issue so the error can be ignored.