When a player joins and immediately leaves the game before a function is
called by minetest.after() in mods/PLAYER/wieldview/init.lua, it gets an
invalidated player object. This results in the player methods returning
nil (since Minetest 5.2); perhaps surprisingly, the player is not nil.
Not checking that the result of player:get_pos() is not nil could lead
to a server crash if a client crashed when joining. It has been reported
that a syntax error in a client side mod was enough to trigger that.
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