As additional information, this doesn't seem to happen with our level 1 quarries/pump jacks. We have them set to go up to level 10, each with 33 storage slots.
After reloading the plugin, now some quarries and pumpjacks that were having this issue aren't, and now a level 1 pump jack is now experiencing this issue. Below is the exception for the level 1 pump jack:
15:32 [Error] Failed to call hook 'VirtualQuarriesConsoleCommand' on plugin 'VirtualQuarries v2.3.0' (NullReferenceException: Object reference not set to an instance of an object)
at Oxide.Plugins.VirtualQuarries.TryStartQuarry (BasePlayer player, System.Int32 quarryId) [0x000bd] in <a31bc5a151bd4c51b2304bbd6001cc29>:0
at Oxide.Plugins.VirtualQuarries.VirtualQuarriesConsoleCommand (ConsoleSystem+Arg arg) [0x0035e] in <a31bc5a151bd4c51b2304bbd6001cc29>:0
at Oxide.Plugins.VirtualQuarries.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x007b6] in <a31bc5a151bd4c51b2304bbd6001cc29>:0
at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <d431e37658ba4727b7490299d12dfc44>:0
at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <157a94ee66ab4a7991faecd1eb84be3b>:0
at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <157a94ee66ab4a7991faecd1eb84be3b>:0