don't create components for entities that don't exist

src/EntityComponentManager.cc

@@ -478,6 +478,15 @@ ComponentKey EntityComponentManager::CreateComponentImplementation(
     const Entity _entity, const ComponentTypeId _componentTypeId,
     const components::BaseComponent *_data)
 {
+  // make sure the entity exists
+  if (!this->HasEntity(_entity))
+  {
+    ignerr << "Trying to create a component of type [" << _componentTypeId
+      << "] attached to entity [" << _entity << "], but this entity does not "
+      << "exist. This create component request will be ignored." << std::endl;
+    return ComponentKey();
+  }
+
   // If type hasn't been instantiated yet, create a storage for it
   if (!this->HasComponentType(_componentTypeId))
   {

src/EntityComponentManager_TEST.cc

@@ -433,6 +433,16 @@ TEST_P(EntityComponentManagerFixture, EntitiesAndComponents)
   EXPECT_FALSE(manager.EntityHasComponentType(entity, DoubleComponent::typeId));
   EXPECT_FALSE(manager.EntityHasComponentType(entity2, IntComponent::typeId));
 
+  // Try to add a component to an entity that does not exist
+  EXPECT_FALSE(manager.HasEntity(kNullEntity));
+  EXPECT_FALSE(manager.EntityHasComponentType(kNullEntity,
+        IntComponent::typeId));
+  EXPECT_EQ(ComponentKey(), manager.CreateComponent<IntComponent>(kNullEntity,
+        IntComponent(123)));
+  EXPECT_FALSE(manager.HasEntity(kNullEntity));
+  EXPECT_FALSE(manager.EntityHasComponentType(kNullEntity,
+        IntComponent::typeId));
+
   // Remove all entities
   manager.RequestRemoveEntities();
   EXPECT_EQ(3u, manager.EntityCount());

src/systems/diff_drive/DiffDrive.cc

@@ -238,7 +238,6 @@ void DiffDrive::Configure(const Entity &_entity,
     this->dataPtr->limiterAng->SetMaxJerk(maxJerk);
   }
 
-
   double odomFreq = _sdf->Get<double>("odom_publish_frequency", 50).first;
   if (odomFreq > 0)
   {
@@ -324,6 +323,10 @@ void DiffDrive::PreUpdate(const ignition::gazebo::UpdateInfo &_info,
 
   for (Entity joint : this->dataPtr->leftJoints)
   {
+    // skip this entity if it has been removed
+    if (!_ecm.HasEntity(joint))
+      continue;
+
     // Update wheel velocity
     auto vel = _ecm.Component<components::JointVelocityCmd>(joint);
 
@@ -340,6 +343,10 @@ void DiffDrive::PreUpdate(const ignition::gazebo::UpdateInfo &_info,
 
   for (Entity joint : this->dataPtr->rightJoints)
   {
+    // skip this entity if it has been removed
+    if (!_ecm.HasEntity(joint))
+      continue;
+
     // Update wheel velocity
     auto vel = _ecm.Component<components::JointVelocityCmd>(joint);
 
@@ -358,15 +365,15 @@ void DiffDrive::PreUpdate(const ignition::gazebo::UpdateInfo &_info,
   // don't exist.
   auto leftPos = _ecm.Component<components::JointPosition>(
       this->dataPtr->leftJoints[0]);
-  if (!leftPos)
+  if (!leftPos && _ecm.HasEntity(this->dataPtr->leftJoints[0]))
   {
     _ecm.CreateComponent(this->dataPtr->leftJoints[0],
         components::JointPosition());
   }
 
   auto rightPos = _ecm.Component<components::JointPosition>(
       this->dataPtr->rightJoints[0]);
-  if (!rightPos)
+  if (!rightPos && _ecm.HasEntity(this->dataPtr->rightJoints[0]))
   {
     _ecm.CreateComponent(this->dataPtr->rightJoints[0],
         components::JointPosition());