Server crashed when attempting to change skin in unified inventory

[d3wdrxp]

This was the error:

`2022-05-23 13:42:28: ERROR[Main]: ServerError: AsyncErr: ServerThread::run Lua: Runtime error from mod '' in callback on_playerReceiveFields(): ...orlds/world/worldmods/skinsdb/unified_inventory_page.lua:10: attempt to concatenate field 'standard_inv_bg' (a nil value)
2022-05-23 13:42:28: ERROR[Main]: stack traceback:
2022-05-23 13:42:28: ERROR[Main]: 	...orlds/world/worldmods/skinsdb/unified_inventory_page.lua:10: in function 'get_formspec'
2022-05-23 13:42:28: ERROR[Main]: 	....worlds/world/worldmods/unified_inventory/internal.lua:84: in function 'get_formspec'
2022-05-23 13:42:28: ERROR[Main]: 	...worlds/world/worldmods/unified_inventory/internal.lua:270: in function 'set_inventory_formspec'
2022-05-23 13:42:28: ERROR[Main]: 	...worlds/world/worldmods/unified_inventory/api.lua:296: in function 'action'
2022-05-23 13:42:28: ERROR[Main]: 	...worlds/world/worldmods/unified_inventory/callbacks.lua:76: in function <...s/worlds/world/worldmods/unified_inventory/callbacks.lua:59>
2022-05-23 13:42:28: ERROR[Main]: 	...e/multicraft/bin/../builtin/game/register.lua:441: in function <...e/diakinesis/multicraft/bin/../builtin/game/register.lua:425>

[SmallJoker]

1. Update unified_inventory to the newest version
2. Update skinsdb to the newest version
3. Can it still be reproduced? How? I did a local test but couldn't find a problem.