Crash on predecessor tech access

[rubenwardy]

ServerError: AsyncErr: ServerThread::run Lua: Runtime error from mod 'ctw_technologies' in callback on_playerReceiveFields(): ...v/minetest/bin/../games/CTW/mods/ctw_resources/ideas.lua:115: Idea ID ipnet is unknown!
2020-04-18 00:44:40: ERROR[Main]: stack traceback:
2020-04-18 00:44:40: ERROR[Main]:       [C]: in function 'error'
2020-04-18 00:44:40: ERROR[Main]:       ...v/minetest/bin/../games/CTW/mods/ctw_resources/ideas.lua:115: in function 'get_idea'
2020-04-18 00:44:40: ERROR[Main]:       .../bin/../games/CTW/mods/ctw_technologies/technologies.lua:31: in function 'show_technology_form'
2020-04-18 00:44:40: ERROR[Main]:       .../bin/../games/CTW/mods/ctw_technologies/technologies.lua:114: in function <.../bin/../games/CTW/mods/ctw_technologies/technologies.lua:96>
2020-04-18 00:44:40: ERROR[Main]:       /home/ruben/dev/minetest/bin/../builtin/game/register.lua:429: in function </home/ruben/dev/minetest/bin/../builtin/game/register.lua:413>

[SmallJoker]

e4a2c2b