一、报错内容：

[ERROR] [1591260841.508422294]: Skipped loading plugin with error: XML Document '/opt/ros/melodic/share/rqt_virtual_joy/plugin.xml' has no Root Element. This likely means the XML is malformed or missing..

 RosPluginProvider._parse_plugin_xml() plugin file "/opt/ros/melodic/share/rqt_virtual_joy/plugin.xml" in package "rqt_virtual_joy" not found 

RosPluginProvider._parse_plugin_xml() plugin file "/opt/ros/melodic/share/rqt_virtual_joy/plugin.xml" in package "rqt_virtual_joy" not found

这个错误是 rqt_virtual_joy 文件夹里没有plugin.xml文件造成的，所以，在相关目录下添加该文件错误就被解决！

二、解决方法：

　　2.1找到 rqt_virtual_joy 目录

1 cd /opt/ros/melodic/share/rqt_virtual_joy

　　2.2新建一个plugin.xml文件

1 touch plugin.xml   # 执行这个命令后，提示错误没有权限。

　　采用下面一行命令赋予管理员权限：

sudo touch plugin.xml

　　2.3 采用gedit编辑或者vim编辑plugin.xml

sudo gedit plugin.xml

　　2.4 将下列代码拷贝到plugin.xml文件

 1 <library path="src">
 2   <class name="My Plugin" type="rqt_virtual_joy.virtual_joy_module.MyPlugin" base_class_type="rqt_gui_py::Plugin">
 3     <description>
 4       An example Python GUI plugin to create a great user interface.
 5     </description>
 6     <qtgui>
 7 
 8       <group>
 9         <label>Robot Tools</label>
10       </group>
11       <!-- <group>
12         <label>Subgroup</label>
13       </group> -->
14 
15       <label>Virtual Joystick</label>
16       <icon type="file">resource/input-gaming.png</icon>
17       <statustip>Great user interface to provide real value.</statustip>
18     </qtgui>
19   </class>
20 </library>

三、参考

https://answers.ros.org/question/354039/rqt_virtual_joypluginxml-file-is-missing/

https://www.cnblogs.com/long5683/p/13847352.html