Error message

I get the following error message: 

Exception while calling NextTick callback (NullReferenceException: Object reference not set to an instance of an object.)
  at Oxide.Plugins.InjuriesAndDiseases.IsInflictionEntity (Oxide.Plugins.InjuriesAndDiseases+StatusCondition condition, BaseEntity entity) [0x00054] in <724f6caec7be49eea5319401923d9f5e>:0 
  at Oxide.Plugins.InjuriesAndDiseases+<>c__DisplayClass50_0.<InflictByEntity>b__0 (Oxide.Plugins.InjuriesAndDiseases+StatusCondition x) [0x00018] in <724f6caec7be49eea5319401923d9f5e>:0 
  at System.Linq.Enumerable+WhereListIterator`1[TSource].MoveNext () [0x00037] in <77c7c3cb49744ec6a3f31b19ec81dcc4>:0 
  at Oxide.Plugins.InjuriesAndDiseases.InflictByEntity (BasePlayer basePlayer, BaseEntity entity, System.Boolean isMelee) [0x000c1] in <724f6caec7be49eea5319401923d9f5e>:0 
  at Oxide.Plugins.InjuriesAndDiseases+<>c__DisplayClass70_0.<OnEntityTakeDamage>b__0 () [0x00395] in <724f6caec7be49eea5319401923d9f5e>:0 
  at Oxide.Core.OxideMod.OnFrame (System.Single delta) [0x00051] in <beb2b64691c64e2b95b99491bd85442c>:0 
 

Do you happen to know what event caused this to occur? It looks like a player was attacked by something, do you know what they were attacked by?

I'm sorry, I do not know 

now additionally the plugin spams the console with: InvalidOperationException: Nullable object must have a value.

Do you have any plugins that add NPCs that can attack players?

yes, the zombie horde and better npc plugins