Properly level up characters with the character exp virtual item

src/main/java/emu/grasscutter/game/inventory/Inventory.java

@@ -15,6 +15,7 @@ import emu.grasscutter.database.DatabaseHelper;
 import emu.grasscutter.game.GenshinPlayer;
 import emu.grasscutter.game.avatar.AvatarStorage;
 import emu.grasscutter.game.avatar.GenshinAvatar;
+import emu.grasscutter.game.entity.EntityAvatar;
 import emu.grasscutter.net.proto.ItemParamOuterClass.ItemParam;
 import emu.grasscutter.server.packet.send.PacketAvatarEquipChangeNotify;
 import emu.grasscutter.server.packet.send.PacketStoreItemChangeNotify;
@@ -187,6 +188,8 @@ public class Inventory implements Iterable<GenshinItem> {
 				existingItem.save();
 				return existingItem;
 			}
+		} else {
+			return null;
 		}
 		
 		// Set ownership and save to db
@@ -207,6 +210,11 @@ public class Inventory implements Iterable<GenshinItem> {
 	
 	private void addVirtualItem(int itemId, int count) {
 		switch (itemId) {
+			case 101: // Character exp
+				for (EntityAvatar entity : getPlayer().getTeamManager().getActiveTeam()) {
+					getPlayer().getServer().getInventoryManager().upgradeAvatar(player, entity.getAvatar(), count);
+				}
+				break;
 			case 102: // Adventure exp
 				getPlayer().addExpDirectly(count);
 				break;

src/main/java/emu/grasscutter/game/managers/InventoryManager.java

@@ -660,6 +660,19 @@ public class InventoryManager {
 		player.getInventory().removeItem(feedItem, count);
 		
 		// Level up
+		upgradeAvatar(player, avatar, promoteData, expGain);
+	}
+	
+	public void upgradeAvatar(GenshinPlayer player, GenshinAvatar avatar, int expGain) {
+		AvatarPromoteData promoteData = GenshinData.getAvatarPromoteData(avatar.getAvatarData().getAvatarPromoteId(), avatar.getPromoteLevel());
+		if (promoteData == null) {
+			return;
+		}
+		
+		upgradeAvatar(player, avatar, promoteData, expGain);
+	}
+	
+	public void upgradeAvatar(GenshinPlayer player, GenshinAvatar avatar, AvatarPromoteData promoteData, int expGain) {
 		int maxLevel = promoteData.getUnlockMaxLevel();
 		int level = avatar.getLevel();
 		int oldLevel = level;