Unknown blocks crash

[jakimfett]

There's a crash when players connect to an area that has unknown blocks present.
(disabling WiTT allows players to connect just fine)

I triggered it by loading the skinmaker mod, placing the skin maker block, and then disabling the mod, restarting the server, and attempting to reconnect to the place I placed the (now unknown) mod block.

2020-11-03 10:09:39: ERROR[Main]: ServerError: AsyncErr: environment_Step: Runtime error from mod 'witt' in callback environment_Step(): ...home/gameserv/run/minetest/bin/../mods/witt/init.lua:113: attempt to index field '?' (a nil value)                                                                                         
2020-11-03 10:09:39: ERROR[Main]: stack traceback:                                                           
2020-11-03 10:09:39: ERROR[Main]:       ...home/gameserv/run/minetest/bin/../mods/witt/init.lua:113: in function 'describe_node'                                                                                          
2020-11-03 10:09:39: ERROR[Main]:       ...home/gameserv/run/minetest/bin/../mods/witt/init.lua:26: in function 'func'                                                                                                    
2020-11-03 10:09:39: ERROR[Main]:       ...minetest/bin/../builtin/profiler/instrumentation.lua:106: in function '?'                                                                                                      
2020-11-03 10:09:39: ERROR[Main]:       ...meserv/run/minetest/bin/../builtin/game/register.lua:429: in function <...meserv/run/minetest/bin/../builtin/game/register.lua:413>                                            
2020-11-03 10:09:39: ERROR[Main]: stack traceback:    

May be fixed by @JakubVanek's patch:
JakubVanek@1521c10

(need to test this though)