I was working with gazebo to move a car inside the simulation using external nodes. It was all working fine until gazebo crashed and I had to restart the PC.

My approach is I have a plugin attached to the car model, that subscribes to a velocity (Float32) topic. Till now I have tested with several models that used to work before the crash eg.velodyne plugin but the model doesn't move although it has an initial velocity set. When I check rostopic list the topic is not shown until I publish to it form the command line, in that case the rqt_graph shows a topic having data published to it hanging in mid-air without being connected to any of the other gazebo nodes/topics :

What I used to see before is the velocity topic publishing to the gazebo node and to a gazebo_client node which is no longer there.

I think that there is an environmental issue in Gazebo that I am not sure how to approach. I am not sure if this is relevant but, I have been trying to run the command gz topic -i gazebo/link_states for published topics shown by rostopic list but I get no data:

Type: 

Publishers:

Subscribers

and

gz topic --verbose -e /gazebo/link_states 

and I get:

[Msg] Waiting for master.
[Msg] Connected to gazebo master @ http://127.0.0.1:11345
[Msg] Publicized address: 10.7.57.252
[Err] [gz_topic.cc:190] Unable to get message type for topic[/gazebo/link_states]

PS: I am using ROS melodic and Gazebo 9