If you are using version 1.2 or are not sure which version you are using, please read the following message:
I was unable to successfully fix the IllegalStateException and that error can cause corrupted worlds. This update reverts to before 1.2, the update that caused the error. Please update immediately. I will not be responsible for corrupted worlds. By continuing to run version 1.2 or version 1.2_FIXED (which actually did not fix anything), you acknowledge that they are unstable and can cause world corruption.