Gazebo crashes using skid_steer, but works fine with diff_drive

edit

I have a skid steer robot which I'm trying to simulate in Gazebo. In my wheel macro, I have:

    <transmission name="${wheel_prefix}_transmission">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="${wheel_prefix}_wheel_joint">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
        </joint>
        <actuator name="${wheel_prefix}_motor">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>

and in my main urdf I have:

<plugin name="skid_steer_drive_controller" filename="libgazebo_ros_skid_steer_drive.so">
  <updateRate>100.0</updateRate>
  <robotNamespace>/</robotNamespace>
  <leftFrontJoint>front_left_wheel_joint</leftFrontJoint>
  <rightFrontJoint>front_right_wheel_joint</rightFrontJoint>
  <leftRearJoint>back_left_wheel_joint</leftRearJoint>
  <rightRearJoint>back_right_wheel_joint</rightRearJoint>
  <wheelSeparation>${wheelbase}</wheelSeparation>
  <wheelDiameter>${2*wheel_radius}</wheelDiameter>
  <robotBaseFrame>base_link</robotBaseFrame>
  <torque>20</torque>
  <commandName>cmd_vel</topicName>
  <broadcastTF>false</broadcastTF>
</plugin>

If I use a diff drive controller, like this:

<plugin name="differential_drive_controller" filename="libgazebo_ros_diff_drive.so">
  <alwaysOn>true</alwaysOn>
  <updateRate>20</updateRate>
  <leftJoint>front_left_wheel_joint</leftJoint>
  <rightJoint>front_right_wheel_joint</rightJoint>
  <wheelSeparation>${wheelbase}</wheelSeparation>
  <wheelDiameter>${2*wheel_radius}</wheelDiameter>
  <robotBaseFrame>base_link</robotBaseFrame>
  <torque>20</torque>
  <commandTopic>cmd_vel</commandTopic>
  <odometryTopic>odom</odometryTopic>
  <odometryFrame>odom</odometryFrame>
</plugin>

Then things work and I can at least use teleop to move the robot backwards and forwards in Gazebo. Any attempt to use the skid steer plugin and Gazebo crashes immediately. It doesn't ever produce a log file, which is unhelpful, so I have no idea what's causing it to break. If I comment out the transmission elements in my xacro file, then Gazebo loads, but obviously I don't get a cmd_vel topic etc. So I guess it's something in the transmission blocks that needs twiddling.

Is there a way to enable verbose output from Gazebo via the launch file?

<include file="$(find gazebo_ros)/launch/empty_world.launch" />

It doesn't help that the main documentation is out of date, e.g. it suggets <topicName> which is a deprecated tag, so I don't know if I'm setting up correctly.

Here's the stdout:

started roslaunch server http://control:33695/

SUMMARY
========

PARAMETERS
 * /robot_description: <?xml version="1....
 * /rosdistro: kinetic
 * /rosversion: 1.12.12
 * /use_sim_time: True

NODES
  /
    gazebo (gazebo_ros/gzserver)
    gazebo_gui (gazebo_ros/gzclient)
    spawn_urdf (gazebo_ros/spawn_model)

auto-starting new master
process[master]: started with pid [4819]
ROS_MASTER_URI=http://localhost:11311

setting /run_id to 2ed86bbe-18a4-11e8-99e9-78d00420dd75
process[rosout-1]: started with pid [4832]
started core service [/rosout]
process[gazebo-2]: started with pid [4842]
process[gazebo_gui-3]: started with pid [4858]
process[spawn_urdf-4]: started with pid [4866]
[ INFO] [1519395408.516996288]: Finished loading Gazebo ROS API Plugin.
[ INFO] [1519395408.518270471]: waitForService: Service [/gazebo/set_physics_properties] has not been advertised, waiting...
[ INFO] [1519395408.906177783, 0.022000000]: waitForService: Service [/gazebo/set_physics_properties] is now available.
[ INFO] [1519395408.941460375, 0.056000000]: Physics dynamic reconfigure ready.
[ WARN] [1519395409.123089568, 0.149000000]: GazeboRosSkidSteerDrive Plugin (ns = //) missing <commandTopic>, defaults to "cmd_vel"
[ WARN] [1519395409.123128894, 0.149000000]: GazeboRosSkidSteerDrive Plugin (ns = //) missing <odometryTopic>, defaults to "odom"
[ WARN] [1519395409.123145859, 0.149000000]: GazeboRosSkidSteerDrive Plugin (ns = //) missing <odometryFrame>, defaults to "odom"
[ WARN] [1519395409.123172229, 0.149000000]: GazeboRosSkidSteerDrive Plugin (ns = //) missing <covariance_x>, defaults to 0.000100
[ WARN] [1519395409.123191756, 0.149000000]: GazeboRosSkidSteerDrive Plugin (ns = //) missing <covariance_y>, defaults to 0.000100
[ WARN] [1519395409.123209074, 0.149000000]: GazeboRosSkidSteerDrive Plugin (ns = //) missing <covariance_yaw>, defaults to 0.010000
Segmentation fault (core dumped)
[gazebo-2 ...