Outbox reading crash

[fluxionary]

AsyncErr: Lua: Runtime error from mod 'mail' in callback on_playerReceiveFields(): ...t/bin/../worlds/basic_test/worldmods/mail/ui/message.lua:30: attempt to index local 'message' (a nil value)
stack traceback:
	...t/bin/../worlds/basic_test/worldmods/mail/ui/message.lua:30: in function 'show_message'
	...st/bin/../worlds/basic_test/worldmods/mail/ui/events.lua:169: in function 'func'
	/opt/minetest/bin/../builtin/profiler/instrumentation.lua:108: in function </opt/minetest/bin/../builtin/profiler/instrumentation.lua:101>
	/opt/minetest/bin/../builtin/game/register.lua:448: in function </opt/minetest/bin/../builtin/game/register.lua:434>

i sent two pieces of mail. i then went into the sent box. i deleted the second, then tried to read the first.

[S-S-X]

Linking #70 seems appropriate (just tracking nothing else).