builtin: Switch to correct get_x() nullptr convention

builtin/ground_plane_lighting/ground_plane_lighting.cpp

@@ -455,7 +455,7 @@ struct Module: public interface::Module, public ground_plane_lighting::Interface
 	{
 		auto it = m_instances.find(scene_ref);
 		if(it == m_instances.end())
-			throw Exception("get_instance(): Scene does not have gpl");
+			return nullptr;
 		return it->second.get();
 	}
 

builtin/voxelworld/api.h

@@ -128,7 +128,7 @@ namespace voxelworld
 			std::function<void(voxelworld::Instance *instance)> cb)
 	{
 		return access(server, [&](voxelworld::Interface *i){
-			voxelworld::Instance *instance = i->get_instance(scene_ref);
+			voxelworld::Instance *instance = check(i->get_instance(scene_ref));
 			cb(instance);
 		});
 	}

builtin/voxelworld/voxelworld.cpp

@@ -1209,8 +1209,7 @@ struct Module: public interface::Module, public voxelworld::Interface
 	{
 		auto it = m_instances.find(scene_ref);
 		if(it == m_instances.end())
-			throw Exception("get_instance(): Scene does not have a voxel"
-					" world instance");
+			return nullptr;
 		return it->second.get();
 	}
 

builtin/worldgen/api.h

@@ -62,7 +62,7 @@ namespace worldgen
 			std::function<void(worldgen::Instance *instance)> cb)
 	{
 		return access(server, [&](worldgen::Interface *i){
-			worldgen::Instance *instance = i->get_instance(scene_ref);
+			worldgen::Instance *instance = check(i->get_instance(scene_ref));
 			cb(instance);
 		});
 	}

builtin/worldgen/worldgen.cpp

@@ -216,7 +216,7 @@ struct Module: public interface::Module, public Interface
 	{
 		auto it = m_instances.find(scene_ref);
 		if(it == m_instances.end())
-			throw Exception("get_instance(): Scene does not have worldgen");
+			return nullptr;
 		return it->second.get();
 	}