Compile error, PlantEntity for growableError
Error while compiling: StatisticsDB.cs(519,35): error CS0246: The type or namespace name `PlantEntity' could not be found. Are you missing an assembly reference?
Also throw it a check in   SaveData  for  "PlayerDatabase !=null"
Beecause ........ in the hook "unload" there is a race condition............ for a server shutting down.......
Depending on the will of the gods..... "PlayerDatabase" may or may not have unloaded BEFORE  "StatsticsDB".

which means you either loose the save & then gain an exception or.. you might make it.

Merged post

also add:
if (stats==null){return;}
in private void OnPlayerDeath

due to
Failed to call hook 'OnPlayerDie' on plugin 'StatisticsDB v1.2.5' (NullReferenceException: Object reference not set to an instance of an object)
at Oxide.Plugins.StatisticsDB.OnPlayerDie (BasePlayer player, HitInfo info) [0x0006c] in <306fb188f03a4863b212ee630318a341>:0

at Oxide.Plugins.StatisticsDB.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00473] in <306fb188f03a4863b212ee630318a341>:0