Fix issues cleaning up after repeating tasks.

A mass of NullPointerExceptions would be outputted when tasks were stopped. This is resolved by checking if we are still running (indicating a possible scheduler cancel) before telling the scheduler to pull the plug.

Ideally, the entire BungeeCord scheduler should be rewritten. It has quite a few issues which could be avoided with a new system.

proxy/src/main/java/net/md_5/bungee/scheduler/BungeeTask.java

@@ -76,6 +76,11 @@ public class BungeeTask implements Runnable, ScheduledTask
             }
         }
 
-        sched.cancel( this );
+        // We might have been previously running before, but now we aren't.
+        // If we weren't already cancelled, then cancel ourselves.
+        if ( running.get() )
+        {
+            sched.cancel( this );
+        }
     }
 }