NRE in RemoveSalvage

From rogueticket-607:

```
Uploading Crash Report
NullReferenceException: Object reference not set to an instance of an object
at LootMagnet.Helper.RemoveSalvage (BattleTech.SalvageDef holdbackDef) [0x00024] in :0
at LootMagnet.UIHelper.AcceptAction (BattleTech.UI.AAR_SalvageScreen salvageScreen, System.Int32 reputationModifier) [0x00097] in :0
at LootMagnet.UIHelper+<>c__DisplayClass2_0.g__acceptAction|0 () [0x00000] in :0
at BattleTech.UI.GenericPopup+<>c__DisplayClass52_0.b__1 () [0x00031] in <4184af8dbeb44635831353f4d349631c>:0
at UnityEngine.Events.InvokableCall.Invoke () [0x00011] in <9a6ceb7f013a4a599806f492fc789483>:0
at UnityEngine.Events.UnityEvent.Invoke () [0x00023] in <9a6ceb7f013a4a599806f492fc789483>:0
at BattleTech.UI.HBSButton.OnPointerUp (UnityEngine.EventSystems.PointerEventData eventData) [0x0006d] in <4184af8dbeb44635831353f4d349631c>:0
at UnityEngine.EventSystems.ExecuteEvents.Execute (UnityEngine.EventSystems.IPointerUpHandler handler, UnityEngine.EventSystems.BaseEventData eventData) [0x00008] in <689405fdf241479fa6250af7e4b0368a>:0
at UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) [0x00070] in <689405fdf241479fa6250af7e4b0368a>:0
UnityEngine.DebugLogHandler:Internal_LogException(Exception, Object)
UnityEngine.DebugLogHandler:LogException(Exception, Object)
UnityEngine.Logger:LogException(Exception, Object)
UnityEngine.Debug:LogException(Exception)
UnityEngine.EventSystems.ExecuteEvents:Execute(GameObject, BaseEventData, EventFunction`1)
UnityEngine.EventSystems.StandaloneInputModule:ProcessMousePress(MouseButtonEventData)
UnityEngine.EventSystems.StandaloneInputModule:ProcessMouseEvent(Int32)
UnityEngine.EventSystems.StandaloneInputModule:ProcessMouseEvent()
UnityEngine.EventSystems.StandaloneInputModule:Process()
UnityEngine.EventSystems.EventSystem:Update()

(Filename: Line: 0)
```

Start by locating LootMagnet.Helper.RemoveSalvage and trace its call from LootMagnet.UIHelper.AcceptAction, using the supplied stack trace as the entry point. Reproduce accepting salvage with a holdback and identify the null value; done means that flow no longer raises the reported NullReferenceException.

Written by the indexing model from the issue text.