Failed to call hook 'CanBuild' on plugin 'EntityLimit v2.1.2' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.EntityLimit.CheckBuild (BasePlayer player, System.String fullName) [0x0000d] in <29007ca332ab4a14b19eac7eda3a807a>:0 
  at Oxide.Plugins.EntityLimit.CanBuild (Planner planner, Construction entity, Construction+Target target) [0x00007] in <29007ca332ab4a14b19eac7eda3a807a>:0 
  at Oxide.Plugins.EntityLimit.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x001db] in <29007ca332ab4a14b19eac7eda3a807a>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <9882f28dc2204b4dba514a9ad18f5042>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <9882f28dc2204b4dba514a9ad18f5042>:0

What you are trying to build? + Attach your config

I've only seen this error once.
if there are many of them, I will let you know.

together with another plugin, at the same time

(12:04:27) | Failed to call hook 'CanBuild' on plugin 'EntityLimit v2.1.2' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.EntityLimit.CheckBuild (BasePlayer player, System.String fullName) [0x0000d] in <02ebda30a2d6408a9c53e0e79ca20852>:0 
  at Oxide.Plugins.EntityLimit.CanBuild (Planner planner, Construction entity, Construction+Target target) [0x00007] in <02ebda30a2d6408a9c53e0e79ca20852>:0 
  at Oxide.Plugins.EntityLimit.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x001db] in <02ebda30a2d6408a9c53e0e79ca20852>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <50629aa0e75d4126b345d8d9d64da28d>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <50629aa0e75d4126b345d8d9d64da28d>:0
(12:04:27) | Failed to call hook 'CanBuild' on plugin 'NoEscape v2.1.35' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.NoEscape.CanRaidCommand (BasePlayer player, System.String command) [0x0000b] in <02ebda30a2d6408a9c53e0e79ca20852>:0 
  at Oxide.Plugins.NoEscape.CanDo (System.String command, BasePlayer player) [0x00000] in <02ebda30a2d6408a9c53e0e79ca20852>:0 
  at Oxide.Plugins.NoEscape.CanBuild (Planner plan, Construction prefab) [0x00007] in <02ebda30a2d6408a9c53e0e79ca20852>:0 
  at Oxide.Plugins.NoEscape.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00947] in <02ebda30a2d6408a9c53e0e79ca20852>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <50629aa0e75d4126b345d8d9d64da28d>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <50629aa0e75d4126b345d8d9d64da28d>:0