A tank was called by a checker. the plane arrived , but the tank did not drop.

Exception while calling NextTick callback (NullReferenceException: )
  at (wrapper managed-to-native) UnityEngine.Component.get_gameObject(UnityEngine.Component)
  at Oxide.Plugins.BradleyDrops.BradleySignalThrown (BasePlayer player, SupplySignal entity, Item signal) [0x005f4] in <7f11e14077d44b96bbb4577a450cd964>:0
  at Oxide.Plugins.BradleyDrops+<>c__DisplayClass39_0.<OnExplosiveThrown>b__0 () [0x00000] in <7f11e14077d44b96bbb4577a450cd964>:0
  at Oxide.Core.OxideMod.OnFrame (System.Single delta) [0x00051] in <18d4f19bbc844191b11ed9e69284d09b>:0

Do you have any other plugins which act on Supply Signals?

You could also try deleting your config and testing with default config. If it works then there might be an error in your config.

To help further you could start by uploading your config file here.