SPIGOT-5122: Skip world#notify if sign has no world.

nms-patches/TileEntitySign.patch

@@ -107,3 +107,12 @@
      }
  
      public EnumColor getColor() {
+@@ -126,7 +178,7 @@
+         if (enumcolor != this.getColor()) {
+             this.color = enumcolor;
+             this.update();
+-            this.world.notify(this.getPosition(), this.getBlock(), this.getBlock(), 3);
++            if (this.world != null) this.world.notify(this.getPosition(), this.getBlock(), this.getBlock(), 3); // CraftBukkit - skip notify if world is null (SPIGOT-5122)
+             return true;
+         } else {
+             return false;