Error message after reload

NullReferenceException: Object reference not set to an instance of an object
  at Oxide.Plugins.MonumentsWatcher.OnEntityExit (BaseEntity entity, Oxide.Plugins.MonumentsWatcher+MonumentWatcher watcher, System.String reason) [0x00000] in <e4b7ca948f7642199142d4950a4672ae>:0 
  at Oxide.Plugins.MonumentsWatcher+MonumentWatcher.ClearEntities () [0x0005d] in <e4b7ca948f7642199142d4950a4672ae>:0 
  at Oxide.Plugins.MonumentsWatcher+MonumentWatcher.OnDestroy () [0x00000] in <e4b7ca948f7642199142d4950a4672ae>:0 
UnityEngine.Object:DestroyImmediate(Object, Boolean)
UnityEngine.Object:DestroyImmediate(Object)
Oxide.Plugins.MonumentsWatcher:ClearWatchers()
Oxide.Plugins.MonumentsWatcher:Unload()
Oxide.Plugins.MonumentsWatcher:DirectCallHook(String, Object&, Object[])
Oxide.Plugins.CSharpPlugin:InvokeMethod(HookMethod, Object[])
Oxide.Core.Plugins.CSPlugin:OnCallHook(String, Object[])
Oxide.Core.Plugins.Plugin:CallHook(String, Object[])
Oxide.Plugins.CSharpPlugin:HandleRemovedFromManager(PluginManager)
Oxide.Core.Plugins.PluginManager:RemovePlugin(Plugin)
Oxide.Core.OxideMod:UnloadPlugin(String)
Oxide.Plugins.<>c__DisplayClass20_0:<Load>b__0(Boolean)
Oxide.Plugins.CompilableFile:Compile(Action`1)
Oxide.Plugins.CSharpPluginLoader:Load(CompilablePlugin)
Oxide.Plugins.CSharpPluginLoader:Reload(String, String)
Oxide.Core.OxideMod:ReloadPlugin(String)
Oxide.Game.Rust.RustCore:ReloadCommand(IPlayer, String, String[])
System.Reflection.RuntimeMethodInfo:InternalInvoke(RuntimeMethodInfo, Object, Object[], Exception&)
System.Reflection.RuntimeMethodInfo:Invoke(Object, BindingFlags, Binder, Object[], CultureInfo)
System.Reflection.MethodBase:Invoke(Object, Object[])
Oxide.Core.Plugins.CSPlugin:InvokeMethod(HookMethod, Object[])
Oxide.Core.Plugins.CSPlugin:OnCallHook(String, Object[])
Oxide.Core.Plugins.Plugin:CallHook(String, Object[])
Oxide.Core.Plugins.<>c__DisplayClass109_0:<AddCovalenceCommand>b__0(IPlayer, String, String[])
Oxide.Core.Plugins.Plugin:CovalenceCommandCallback(IPlayer, String, String[])
Oxide.Game.Rust.Libraries.Covalence.<>c__DisplayClass7_0:<RegisterCommand>b__0(Arg)
ConsoleSystem:Internal(Arg)
ConsoleSystem:Run(Option, String, Object[])
Facepunch.RCon:OnCommand(Command)
Facepunch.RCon:Update()
ServerMgr:DoTick()

Only the following 5 plugins were used. 
(Possibly this is also related to the problem that Zone Status 0.1.3 has problems).

01 "ZoneStatus" (0.1.3) by IIIaKa (0.01s / 136 KB) - ZoneStatus.cs
 02 "Image Library" (2.0.62) by Absolut & K1lly0u (0.02s / 5 MB) - ImageLibrary.cs
 03 "Monuments Watcher" (0.1.3) by IIIaKa (0.01s / 248 KB) - MonumentsWatcher.cs
 04 "Advanced Status" (0.1.14) by IIIaKa (0.00s / 8 KB) - AdvancedStatus.cs
 05 "Zone Manager" (3.1.4) by k1lly0u (0.00s / 0 B) - ZoneManager.cs

The issue was with the OnEntityDeath hook (death cannot be canceled), it somehow wasn't tracking all deaths. Consequently, entities weren't being removed from the monument list, and upon unloading the plugin, the list contains null entities.
I had to change it to OnEntityKill, which triggers on all deaths, but unfortunately, another plugin can cancel the death, causing the entity to be removed from the list even if it doesn't die.

Try redownloading the file.

Changed Status from Pending to Closed

Changed Fixed In to 0.1.3

If the issue persists, feel free to report it in this thread.

But now another error message appears 

NullReferenceException: Object reference not set to an instance of an object
  at Oxide.Plugins.MonumentsWatcher.OnEntityExit (BaseEntity entity, Oxide.Plugins.MonumentsWatcher+MonumentWatcher watcher, System.String reason) [0x00000] in <d54c6c1bbf3f4947924e22d6a8d4b917>:0 
  at Oxide.Plugins.MonumentsWatcher+MonumentWatcher.ClearEntities () [0x0005d] in <d54c6c1bbf3f4947924e22d6a8d4b917>:0 
  at Oxide.Plugins.MonumentsWatcher+MonumentWatcher.OnDestroy () [0x00000] in <d54c6c1bbf3f4947924e22d6a8d4b917>:0 
UnityEngine.Object:DestroyImmediate(Object, Boolean)
UnityEngine.Object:DestroyImmediate(Object)
Oxide.Plugins.MonumentsWatcher:ClearWatchers()
Oxide.Plugins.MonumentsWatcher:Unload()
Oxide.Plugins.MonumentsWatcher:DirectCallHook(String, Object&, Object[])
Oxide.Plugins.CSharpPlugin:InvokeMethod(HookMethod, Object[])
Oxide.Core.Plugins.CSPlugin:OnCallHook(String, Object[])
Oxide.Core.Plugins.Plugin:CallHook(String, Object[])
Oxide.Plugins.CSharpPlugin:HandleRemovedFromManager(PluginManager)
Oxide.Core.Plugins.PluginManager:RemovePlugin(Plugin)
Oxide.Core.OxideMod:UnloadPlugin(String)
Oxide.Plugins.CSharpPluginLoader:<CompileAssembly>b__24_0(Compilation)
Oxide.Plugins.Compilation:<Completed>b__17_0()
Oxide.Core.OxideMod:OnFrame(Single)
Oxide.Core.Unity.UnityScript:Update()

Changed Status from Closed to Pending

Does this message occur during plugin unload/reload?
Try completely removing the plugin and all its files, then reinstalling it.

Hi, try to update to 0.1.4

Changed Status from Pending to Closed

Changed Fixed In from 0.1.3 to 0.1.4