add a error message

src/units/unit.cpp

@@ -33,6 +33,7 @@
 #include "game_events/manager.hpp" // for add_events
 #include "game_version.hpp"
 #include "gettext.hpp" // for N_
+#include "gui/dialogs/message.hpp"
 #include "lexical_cast.hpp"
 #include "log.hpp"                       // for LOG_STREAM, logger, etc
 #include "map/map.hpp"                   // for gamemap
@@ -1791,9 +1792,16 @@ int unit::resistance_against(const std::string& damage_name,bool attacker,const
 	int res = opp_weapon ? movement_type_.resistance_against(*opp_weapon) : movement_type_.resistance_against(damage_name);
 
 	unit_ability_list resistance_abilities = get_abilities_weapons("resistance",loc, weapon, opp_weapon);
+	bool error_message = false;
 	utils::erase_if(resistance_abilities, [&](const unit_ability& i) {
+		if(!error_message && (*i.ability_cfg)["max_value"].empty()){
+			error_message = true;
+		}
 		return ((*i.ability_cfg)["max_value"].empty() || !resistance_filter_matches(*i.ability_cfg, attacker, damage_name, 100-res));
 	});
+	if(error_message){
+		gui2::show_error_message(_("One of [resistance] abilities don't have max_value attribute and is not valid."));
+	}
 
 	if(!resistance_abilities.empty()) {
 		unit_abilities::effect resist_effect(resistance_abilities, 100-res);