Create urdf file "check_urdf" cannot check the urdf file

edit

when i key in check_urdf mybot.urdf, the terminal shows the message like that :

Error:   Error reading end tag.
 at line 72 in /build/urdfdom-UJ3kd6/urdfdom-0.4.1/urdf_parser/src/model.cpp
ERROR: Model Parsing the xml failed

my system is Ubuntu 16.04, ROS version is kinetic

mybot.urdf

<robot name="turtlebot_nano">
    <link name="base_link" />
    <link name="right_back_wheel_link" />
    <link name="left_back_wheel_link" />
    <link name="right_front_wheel_link" />
    <link name="left_front_wheel_link" />
    <link name="lidar_link" />
    <link name="camera_link" />


    <joint name="joint1" type="continuous">
        <parent link="base_link"/>
        <child link="right_back_wheel_link">
    </joint>

    <joint name="joint2" type="continuous">
        <parent link="base_link"/>
        <child link="left_back_wheel_link"/>
    </joint>

    <joint name="joint3" type="continuous">
        <parent link="base_link"/>
        <child link="right_front_wheel_link"/>
    </joint>

    <joint name="joint4" type="continuous">
        <parent link="base_link"/>
        <child link="left_front_wheel_link"/>
    </joint>

    <joint name="joint5" type="continuous">
        <parent link="base_link"/>
        <child link="lidar_link"/>
    </joint>

    <joint name="joint6" type="continuous">
        </parent link="base_link"/>
        <child link="camera_link">
    </joint>
</robot>