Error caused by bag stack movement trickery

[fluxionary]

1. put a bag into an empty bag slot. this won't work w/ a bag w/ contents.
2. pick up the bag with the mouse, but don't put it anywhere yet
3. while still holding the bag, click the button to open the bag's inventory
4. place the bag in your main inventory.
5. while the bag inventory is still open, enter some text in the search field, and submit the search.

AsyncErr: Lua: Runtime error from mod 'unified_inventory' in callback on_playerReceiveFields(): /opt/minetest/bin/../mods/unified_inventory/bags.lua:67: attempt to perform arithmetic on local 'slots' (a nil value)
stack traceback:
    /opt/minetest/bin/../mods/unified_inventory/bags.lua:67: in function 'get_formspec'
    /opt/minetest/bin/../mods/unified_inventory/internal.lua:309: in function 'get_formspec'
    /opt/minetest/bin/../mods/unified_inventory/internal.lua:334: in function 'set_inventory_formspec'
    /opt/minetest/bin/../mods/unified_inventory/internal.lua:418: in function 'apply_filter'
    /opt/minetest/bin/../mods/unified_inventory/callbacks.lua:79: in function 'receive_fields_searchbox'
    /opt/minetest/bin/../mods/unified_inventory/callbacks.lua:96: 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>

[SmallJoker]

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