Fix bug where returning reference to {} would crash

src/TechTreeImproved.cpp

@@ -119,6 +119,6 @@ const std::vector<BuildDescription> & TechTreeImproved::HowToBuild(sc2::UnitType
     else
     {
         std::cout << "No information about unit type " << sc2::UnitTypeToName(unit) << " (" << static_cast<int>(unit) << ")" << std::endl;
-        return {};
+        return empty;
     }
 }
\ No newline at end of file

src/TechTreeImproved.h

@@ -25,6 +25,9 @@ class TechTreeImproved
     std::map<sc2::UNIT_TYPEID, std::vector<BuildDescription>> producer_to_data;
     std::map<sc2::UNIT_TYPEID, std::vector<BuildDescription>> result_to_data;
 
+    // If there is no BuildDescription for a given type, a reference to tihs list is returned.
+    const std::vector<BuildDescription> empty {};
+
     void parse_unit(nlohmann::json::iterator it);
 public:
     TechTreeImproved();