Load collection from db

src/engine/db/handlers/dbBuildingHandler.java

@@ -28,6 +28,7 @@ import java.time.LocalDateTime;
 import java.time.ZoneId;
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.concurrent.ConcurrentHashMap;
 
 public class dbBuildingHandler extends dbHandlerBase {
 
@@ -435,6 +436,11 @@ public class dbBuildingHandler extends dbHandlerBase {
             while (rs.next()) {
                 BuildingFriends friend = new BuildingFriends(rs);
 
+                // Create map if it does not yet exist
+
+                if (!BuildingManager._buildingFriends.containsKey(friend.buildingUID))
+                    BuildingManager._buildingFriends.put(friend.buildingUID, new ConcurrentHashMap<>());
+
                 switch (friend.friendType) {
                     case 7:
                         BuildingManager._buildingFriends.get(friend.buildingUID).put(friend.playerUID, friend);