Fix bug in reload/re-init behaviour

Processing instances were not being unloaded properly when Processing.reload() was called, since the `processingInstances` variable is no longer in scope at that point (since the post-v1.4.1 refactor). 
this check failing should not be possible. I'd rather not have this test in here plus a unit test that does some sketch building/reloading to stress test the init() function.