Fix potential NPE when unloading Visibility Status

Author: ZockerAxel

src/eu/endercentral/crazy_advancements/advancement/Advancement.java

@@ -436,6 +436,7 @@ public boolean getVisibilityStatus(Player player) {
 	 * @param player The Player to Unload Visibility
 	 */
 	public void unloadVisibilityStatus(Player player) {
+		if(savedVisibilityStatus == null) return;
 		savedVisibilityStatus.remove(player.getUniqueId().toString());
 	}