Don't allow world unloading while ticking worlds (fixes #8080) (#8081)

patches/server/Throw-exception-on-world-create-while-being-ticked.patch

@@ -46,3 +46,11 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
          Validate.notNull(creator, "Creator may not be null");
  
          String name = creator.name();
+@@ -0,0 +0,0 @@ public final class CraftServer implements Server {
+ 
+     @Override
+     public boolean unloadWorld(World world, boolean save) {
++        Preconditions.checkState(!this.console.isIteratingOverLevels, "Cannot unload a world while worlds are being ticked"); // Paper
+         if (world == null) {
+             return false;
+         }