crash Forward without mail selected

[Bastrabun]

Sorry if we throw you dead with issues right now, but testing is our way to appreciate the effort you put into it ;)

Repro:

1. Write yourself a mail or have one in the inbox
2. delete it
3. push the forward button

Attempt1:

2023-05-01 19:43:20: ACTION[Server]: [yl_commons] formspec "mail:inbox": Boris submitted {"delete":"L\u00f6schen","filter":"","sortdirection":"1","sortfield":"2"}
2023-05-01 19:43:20: ACTION[Server]: [yl_commons] formspec "mail:inbox": Boris submitted {"delete":"L\u00f6schen","filter":"","sortdirection":"1","sortfield":"2"}
2023-05-01 19:43:20: ACTION[Server]: [yl_commons] formspec "mail:inbox": Boris submitted {"delete":"L\u00f6schen","filter":"","sortdirection":"1","sortfield":"2"}
2023-05-01 19:43:21: ACTION[Server]: [yl_commons] formspec "mail:inbox": Boris submitted {"filter":"","forward":"Weiter","sortdirection":"1","sortfield":"2"}
2023-05-01 19:43:21: ACTION[Main]: Server: Shutting down
2023-05-01 19:43:23: ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod 'mail' in callback on_playerReceiveFields(): .../5.6.1/Minetest_test/bin/../mods/mail_mod/ui/message.lua:83: attempt to index local 'message' (a nil value)
2023-05-01 19:43:23: ERROR[Main]: stack traceback:
2023-05-01 19:43:23: ERROR[Main]: 	.../5.6.1/Minetest_test/bin/../mods/mail_mod/ui/message.lua:83: in function 'forward'
2023-05-01 19:43:23: ERROR[Main]: 	...t/5.6.1/Minetest_test/bin/../mods/mail_mod/ui/events.lua:220: in function 'func'
2023-05-01 19:43:23: ERROR[Main]: 	...inetest_test/bin/../builtin/profiler/instrumentation.lua:107: in function <...inetest_test/bin/../builtin/profiler/instrumentation.lua:100>
2023-05-01 19:43:23: ERROR[Main]: 	.../mt/5.6.1/Minetest_test/bin/../builtin/game/register.lua:432: in function <.../mt/5.6.1/Minetest_test/bin/../builtin/game/register.lua:418>

Attempt2:

2023-05-01 19:47:43: ACTION[Server]: [yl_commons] formspec "mail:compose": Boris submitted {"bcc":"","body":"asdasd","cc":"","send":"Senden","subject":"asdasd","to":"Boris"}
2023-05-01 19:47:43: ACTION[Server]: [mail] "Boris" send mail to "Boris" (CC:  - BCC: ) with subject "asdasd" and body "asdasd"
2023-05-01 19:47:46: ACTION[Server]: [yl_commons] formspec "mail:inbox": Boris submitted {"filter":"","forward":"Weiter","sortdirection":"1","sortfield":"1"}
2023-05-01 19:47:47: ACTION[Server]: [yl_commons] formspec "mail:inbox": Boris submitted {"filter":"","inbox":"CHG:2:3","sortdirection":"1","sortfield":"1"}
2023-05-01 19:47:49: ACTION[Server]: [yl_commons] lua is using 342.5 MB
2023-05-01 19:47:49: ACTION[Server]: [yl_commons] formspec "mail:inbox": Boris submitted {"delete":"L\u00f6schen","filter":"","sortdirection":"1","sortfield":"1"}
2023-05-01 19:47:52: ACTION[Server]: [yl_commons] formspec "mail:inbox": Boris submitted {"filter":"","forward":"Weiter","sortdirection":"1","sortfield":"1"}
2023-05-01 19:47:52: ACTION[Main]: Server: Shutting down
2023-05-01 19:47:53: ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod 'mail' in callback on_playerReceiveFields(): .../5.6.1/Minetest_test/bin/../mods/mail_mod/ui/message.lua:83: attempt to index local 'message' (a nil value)
2023-05-01 19:47:53: ERROR[Main]: stack traceback:
2023-05-01 19:47:53: ERROR[Main]: 	.../5.6.1/Minetest_test/bin/../mods/mail_mod/ui/message.lua:83: in function 'forward'
2023-05-01 19:47:53: ERROR[Main]: 	...t/5.6.1/Minetest_test/bin/../mods/mail_mod/ui/events.lua:220: in function 'func'
2023-05-01 19:47:53: ERROR[Main]: 	...inetest_test/bin/../builtin/profiler/instrumentation.lua:107: in function <...inetest_test/bin/../builtin/profiler/instrumentation.lua:100>
2023-05-01 19:47:53: ERROR[Main]: 	.../mt/5.6.1/Minetest_test/bin/../builtin/game/register.lua:432: in function <.../mt/5.6.1/Minetest_test/bin/../builtin/game/register.lua:418>

For my reference: 4376

[S-S-X]

Linking #70 as this seems like another thing where it would be helpful. Just a link, not necessarily relevant with fixing this issue.

[Athozus]

Thanks for reporting that, it should be trivial. I will work on it tomorrow because I think it's also related to #80 and #81. I should add a verification to check if selected messages are valid.