Fix plugin conversion error message (#2094)

This fixes the error

[Err] [Conversions.cc:1791] Tried to convert SDF [world] into [plugin]

The error comes from AddSystem passing an sdf::ElementPtr that corresponds to a <world> tag instead of a <plugin> tag when it's optional _sdf element is empty, which occurs mostly in tests when AddSystem is used to add Relay plugin which doesn't have an associated SDFormat file.


---------

Signed-off-by: Addisu Z. Taddese <addisu@openrobotics.org>

src/SimulationRunner.cc

@@ -453,13 +453,23 @@ void SimulationRunner::PublishStats()
     this->rootClockPub.Publish(clockMsg);
 }
 
+namespace {
+
+// Create an sdf::ElementPtr that corresponds to an empty `<plugin>` element.
+sdf::ElementPtr createEmptyPluginElement()
+{
+  auto plugin = std::make_shared<sdf::Element>();
+  sdf::initFile("plugin.sdf", plugin);
+  return plugin;
+}
+}
 //////////////////////////////////////////////////
 void SimulationRunner::AddSystem(const SystemPluginPtr &_system,
       std::optional<Entity> _entity,
       std::optional<std::shared_ptr<const sdf::Element>> _sdf)
 {
   auto entity = _entity.value_or(worldEntity(this->entityCompMgr));
-  auto sdf = _sdf.value_or(this->sdfWorld->Element());
+  auto sdf = _sdf.value_or(createEmptyPluginElement());
   this->systemMgr->AddSystem(_system, entity, sdf);
 }
 
@@ -470,7 +480,7 @@ void SimulationRunner::AddSystem(
       std::optional<std::shared_ptr<const sdf::Element>> _sdf)
 {
   auto entity = _entity.value_or(worldEntity(this->entityCompMgr));
-  auto sdf = _sdf.value_or(this->sdfWorld->Element());
+  auto sdf = _sdf.value_or(createEmptyPluginElement());
   this->systemMgr->AddSystem(_system, entity, sdf);
 }
 

src/TestFixture_TEST.cc

@@ -42,7 +42,7 @@ class TestFixtureTest : public InternalFixture<::testing::Test>
     EXPECT_EQ(worldEntity, _entity);
 
     ASSERT_NE(nullptr, _sdf);
-    EXPECT_EQ("world", _sdf->GetName());
+    EXPECT_EQ("plugin", _sdf->GetName());
 
     EXPECT_NE(kNullEntity, _ecm.EntityByComponents(components::Name("box")));
     EXPECT_NE(kNullEntity, _ecm.EntityByComponents(components::Name("sphere")));