Make it RUN under Minetest 5.1.1!!! :)

mods/ENTITIES/mcl_item_entity/init.lua

@@ -107,7 +107,9 @@ minetest.register_globalstep(function(dtime)
 
 			local pos = player:get_pos()
 
-			if tick == true and pool[name] > 0 then
+			local pool_name = pool[name]
+
+			if tick == true and pool_name and pool_name > 0 then
 				minetest.sound_play("item_drop_pickup", {
 					pos = pos,
 					gain = 0.7,

mods/ENVIRONMENT/mcl_moon/init.lua

@@ -55,10 +55,16 @@ minetest.register_globalstep(function(dtime)
 	for p=1, #players do
 		if players[p].set_moon then
 			players[p]:set_moon(moon_arg)
+		else
+			-- TODO: use old sky api
 		end
 	end
 end)
 
 minetest.register_on_joinplayer(function(player)
-	player:set_moon({texture = get_moon_texture(), scale=3.75})
+	if player.set_moon then
+		player:set_moon({texture = get_moon_texture(), scale=3.75})
+	else
+		-- TODO: use old sky api
+	end
 end)

mods/PLAYER/mcl_player/init.lua

@@ -90,6 +90,11 @@ end
 
 local function set_texture(player, index, texture)
 	local textures = player_textures[player:get_player_name()]
+	if not textures then
+		-- TODO: Check if minetest.register_on_joinplayer() shouldn't really work in minetest 5.1.1?
+		player_textures[player:get_player_name()] = {"blank.png", "blank.png", "blank.png"}
+		textures = player_textures[player:get_player_name()]
+	end
 	textures[index] = texture
 	player:set_properties({textures = textures})
 end

mods/PLAYER/mcl_playerplus/init.lua

@@ -277,6 +277,11 @@ minetest.register_globalstep(function(dtime)
 		local fly_node = minetest.get_node({x = fly_pos.x, y = fly_pos.y - 0.5, z = fly_pos.z}).name
 		local elytra = mcl_playerplus.elytra[player]
 
+		if not elytra then
+			mcl_playerplus.elytra[player] = {}
+			elytra = mcl_playerplus.elytra[player]
+		end
+
 		elytra.active = player:get_inventory():get_stack("armor", 3):get_name() == "mcl_armor:elytra"
 			and not player:get_attach()
 			and (elytra.active or control.jump and player_velocity.y < -6)
@@ -399,7 +404,8 @@ minetest.register_globalstep(function(dtime)
 		-- Update jump status immediately since we need this info in real time.
 		-- WARNING: This section is HACKY as hell since it is all just based on heuristics.
 
-		if mcl_playerplus_internal[name].jump_cooldown > 0 then
+		local mcl_playerplus_internal_name = mcl_playerplus_internal[name]
+		if mcl_playerplus_internal_name and mcl_playerplus_internal_name.jump_cooldown > 0 then
 			mcl_playerplus_internal[name].jump_cooldown = mcl_playerplus_internal[name].jump_cooldown - dtime
 		end
 
@@ -614,6 +620,9 @@ minetest.register_globalstep(function(dtime)
 		end
 
 		-- Update internal values
+		if not mcl_playerplus_internal[name] then
+			mcl_playerplus_internal[name] = {}
+		end
 		mcl_playerplus_internal[name].lastPos = pos
 
 	end