Error on Melon Loader when opening BTD6

[QuanNguyenIU]

I caught the following error on Melon Loader when opening BTD6, and the mod could not be activated.
Melon Loader, the mod and BTD6 all upgraded to latest version.

[16:40:58.919] [BloonsTD6_Mod_Helper] Registering ModContent for Frankenstein Hero...
[16:40:59.056] [BloonsTD6_Mod_Helper] Failed to ApplyUpgrade FrankensteinHero Level 17 to TowerModel FrankensteinHero-FrankensteinHero 17
[16:40:59.059] [Frankenstein_Hero] Failed to register FrankensteinHero
[16:40:59.143] [BloonsTD6_Mod_Helper] System.NullReferenceException: Object reference not set to an instance of an object.
at FrankensteinHerolevels.Lv17.ApplyUpgrade(TowerModel towerModel) in C:\Users\hubor\Documents\BTD6 Mod Sources\FrankensteinHero\levels.cs:line 460
at BTD_Mod_Helper.Api.Towers.ModTowerHelper.FinalizeTowerModel(ModTower modTower, TowerModel towerModel)
at BTD_Mod_Helper.Api.Towers.ModTower.Register()
at BTD_Mod_Helper.Api.Towers.ModHero.Register()
at BTD_Mod_Helper.Api.Internal.ModContentTask.Coroutine()+MoveNext()