Funny enough this same issue happened to me awhile back..   
"LuaGUI: GUI Error: [string "./MissionEditor/modules/loadLiveries.lua"]:86: bad argument #1 to 'loadstring' (string expected, got nil) 
GUI debug.traceback: stack traceback: 
	[C]: ? 
	[C]: in function 'loadstring' 
	[string "./MissionEditor/modules/loadLiveries.lua"]:86: in function 'loadLiveryZIP' 
	[string "./MissionEditor/modules/loadLiveries.lua"]:110: in function 'loadNameScheme' 
	[string "./MissionEditor/modules/loadLiveries.lua"]:122: in function 'loadSchemes' 
	[string "./MissionEditor/modules/me_payload.lua"]:486: in function 'updateLiveries' 
	[string "./MissionEditor/modules/me_payload.lua"]:761: in function 'update' 
	[string "./MissionEditor/modules/me_loadout.lua"]:533: in function 'updatePayloadWeight' 
	[string "./MissionEditor/modules/me_loadout.lua"]:561: in function 'selectPayload' 
	[string "./MissionEditor/modules/me_loadout.lua"]:840: in function 'selectMissionPayload' 
	[string "./MissionEditor/modules/me_loadout.lua"]:908: in function 'update' 
	[string "./MissionEditor/modules/me_aircraft.lua"]:687: in function 'changeType' 
	[string "./MissionEditor/modules/me_aircraft.lua"]:2153: in function 'onChange' 
	[string "./dxgui/bind/ComboList.lua"]:21: in function 'callback' 
	[string "./dxgui/bind/Widget.lua"]:330: in function <[string "./dxgui/bind/Widget.lua"]:325>"     
"LoadLiveryZIP"   
Is there a zip file in your Harrier's livery folder? If so. remove it.. It's causing havoc because the Sim simply cannot load the textures in the ZIP.   
"DCS World OpenBeta\CoreMods\aircraft\AV8BNA\Liveries\AV8BNA"