Gazebo spawn_sdf_model crash after delete call

[schmittlema]

After spawning a model with the gazebo/spawn_sdf_model then deleting it with gazebo/delete_model any subsequent spawn of a model results in the following error:

gzserver: /usr/include/boost/smart_ptr/shared_ptr.hpp:648: typename boost::detail::sp_member_access::type boost::shared_p ::operator->() const [with T = gazebo::rendering::Scene; typename boost::detail::sp_member_access::type = gazebo::rendering::Scene*]: Assertion `px != 0' failed.

followed by:
ServiceException: unable to connect to service: [Errno 104] Connection reset by peer