On forge there was a problem sometimes, where presumably a join-fork-thread was created with a class-loader that did not have access to all classes, and therefore crashed in a ClassNotFoundException. This fixes this issue.
This uses a workaround that is checking for each chunk-load-event, if that chunk is already present and generated on the world files. The chunk will most likely not be present on the worldfiles right after being generated, because it is usually not saved to disk right away.
