"cannot marshal None unless allow_none is enabled" TypeError: cannot marshal None unless allow_none is enabled

edit

i created a lauch file to launch my robot into gazebo. first i got oid gains error so i added pid gains manually.Then when i run the launch file i am getting the following error

load_parameters: unable to set parameters (last param was [/p=100.0]): cannot marshal None unless allow_none is enabled
Traceback (most recent call last):
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/__init__.py", line 332, in main
    p.start()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/parent.py", line 300, in start
    self.runner.launch()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/launch.py", line 662, in launch
    self._setup()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/launch.py", line 649, in _setup
    self._load_parameters()


    File "/opt/ros/melodic/lib/python2.7/dist-pack

ages/roslaunch/launch.py", line 341, in _load_parameters
    r  = param_server_multi()
  File "/usr/lib/python2.7/xmlrpclib.py", line 1006, in __call__
    return MultiCallIterator(self.__server.system.multicall(marshalled_list))
  File "/usr/lib/python2.7/xmlrpclib.py", line 1243, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1596, in __request
    allow_none=self.__allow_none)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1094, in dumps
    data = m.dumps(params)



    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 660, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 741, in dump_struct
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 660, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 719, in dump_array
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 660, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 664, in dump_nil
    raise TypeError, "cannot marshal None unless allow_none is enabled"
TypeError: cannot marshal None unless allow_none is enabled

and i first launched the gazebo empty world and launched my file to spawn the robot

<?xml version="1.0" encoding="UTF-8"?>

<launch>
    <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find tm_grasp_description)/urdf/tm700_robot.urdf.xacro'" />

    <arg name="x" default="0"/>
    <arg name="y" default="0"/>
    <arg name="z" default="0.5"/>

    <node name="mybot_spawn" pkg="gazebo_ros" type="spawn_model" output="screen"
          args="-urdf -param robot_description -model m2wr -x $(arg x) -y $(arg y) -z $(arg z)" />
    <rosparam file="$(find tm700_moveit_config)/config/gazebo_ros_control.yaml" command="load"/> 
</launch>