解决Skipped loading plugin with error: XML Document '/opt/ros/melodic/share/rqt_virtual_joy/plugin.xml' has no Root Element的问题

1、在ROS官网找到rqt_virtual_joy/plugin.xml，复制文件内容

<library path="src">
  <class name="My Plugin" type="rqt_virtual_joy.virtual_joy_module.MyPlugin" base_class_type="rqt_gui_py::Plugin">
    <description>
      An example Python GUI plugin to create a great user interface.
    </description>
    <qtgui>

      <group>
        <label>Robot Tools</label>
      </group>
      <!-- <group>
        <label>Subgroup</label>
      </group> -->

      <label>Virtual Joystick</label>
      <icon type="file">resource/input-gaming.png</icon>
      <statustip>Great user interface to provide real value.</statustip>
    </qtgui>
  </class>
</library>

2.打开报错中指出的文件缺失的目录/opt/ros/melodic/share/rqt_virtual_joy/，新建文件plugin.xml

3.打开plugin.xml，粘贴第一步复制的代码，保存退出即可。