crash on high xp value

2021-02-21 19:02:45: ERROR[Main]: ServerError: AsyncErr: environment_Step: Runtime error from mod 'missions' in callback environment_Step(): /data/world//worldmods/missions/steps/checkxp.lua:43: attempt to compare nil with number
2021-02-21 19:02:45: ERROR[Main]: stack traceback:
2021-02-21 19:02:45: ERROR[Main]: 	/data/world//worldmods/missions/steps/checkxp.lua:43: in function 'on_step_enter'
2021-02-21 19:02:45: ERROR[Main]: 	/data/world//worldmods/missions/executor.lua:112: in function 'update_mission'
2021-02-21 19:02:45: ERROR[Main]: 	/data/world//worldmods/missions/executor.lua:192: in function 'func'
2021-02-21 19:02:45: ERROR[Main]: 	...ocal/share/minetest/builtin/profiler/instrumentation.lua:106: in function 'globalstep'
2021-02-21 19:02:45: ERROR[Main]: 	/data/world//worldmods/monitoring/builtin/globalstep.lua:32: in function </data/world//worldmods/monitoring/builtin/globalstep.lua:24>
2021-02-21 19:02:45: ERROR[Main]: 	/usr/local/share/minetest/builtin/game/register.lua:429: in function </usr/local/share/minetest/builtin/game/register.lua:413>
2021-02-21 19:02:45: ERROR[Main]: stack traceback:

Start by inspecting worldmods/missions/steps/checkxp.lua at line 43, then trace the call from worldmods/missions/executor.lua. Reproduce the high-XP mission step if possible and verify that the comparison no longer crashes when XP is missing or unusually large.