Fix use on nil value

[BackrndSource]

Im having this issue:

Error running system event handling function for resource esx_thief: citizen:/scripting/lua/scheduler.lua:41: Failed to execute thread: @esx_thief/server/main.lua:47: attempt to compare number with nil
stack traceback:
        @esx_thief/server/main.lua:47: in upvalue 'handler'
        citizen:/scripting/lua/scheduler.lua:219: in function <citizen:/scripting/lua/scheduler.lua:218>
stack traceback:
        [C]: in function 'error'
        citizen:/scripting/lua/scheduler.lua:41: in field 'CreateThreadNow'
        citizen:/scripting/lua/scheduler.lua:218: in function <citizen:/scripting/lua/scheduler.lua:182>

This will fix it