Map handling improvements.

Fixed an NPE occurring under certain circumstances.
Made it possible for maps to exist without having to associate them with a world.

src/main/java/net/minecraft/server/WorldMap.java

@@ -45,7 +45,7 @@ public class WorldMap extends WorldMapBase {
 
             if (least != 0L && most != 0L) {
                 this.uniqueId = new UUID(most, least);
-            }
+
                 CraftWorld world = (CraftWorld) server.getWorld(this.uniqueId);
                 // Check if the stored world details are correct.
                 if (world == null) {
@@ -56,6 +56,7 @@ public class WorldMap extends WorldMapBase {
                     dimension = (byte) world.getHandle().dimension;
                 }
             }
+        }
 
         this.map = dimension;
         // CraftBukkit end
@@ -110,9 +111,13 @@ public class WorldMap extends WorldMapBase {
                     }
                 }
             }
+            /* Perform a second check to see if a matching world was found, this is a necessary
+               change incase Maps are forcefully unlinked from a World and lack a UID.*/
+            if (this.uniqueId != null) {
                 nbttagcompound.setLong("UUIDLeast", this.uniqueId.getLeastSignificantBits());
                 nbttagcompound.setLong("UUIDMost", this.uniqueId.getMostSignificantBits());
             }
+        }
         // CraftBukkit end
         nbttagcompound.a("dimension", this.map);
         nbttagcompound.a("xCenter", this.b);