[Gazebo] child link "base_link" not found (World -> UR5) [closed]

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I am working on a Gazebo simulation of a UR5 with a Robotiq 2f gripper (ROS Melodic + Ubuntu 18.04) and created my xacro file by loading the ur5 description, creating a world link and attaching the world link to the base_link of the robot as follows:

<?xml version="1.0" ?>
<robot name="ur5_2f85" xmlns:xacro="http://ros.org/wiki/xacro">
<xacro:arg name="kinematics_config" default="$(find ur_description)/config/ur5/default_kinematics.yaml"/>

<!-- ROBOT -->
<xacro:include filename="$(find ur_description)/urdf/common.gazebo.xacro"/>
<xacro:include filename="$(find ur_description)/urdf/ur5.urdf.xacro"/>
<xacro:ur5_robot prefix="" joint_limited="true"/>

<!-- WORLD -->
<link name="world"/>

<joint name="world_joint" type="fixed">
    <parent link="world"/>
    <child link="base_link"/>
    <origin xyz="0 0 0" rpy="0 0 0"/>
</joint>

<!-- GRIPPER COUPLING -->
<xacro:include filename="$(find ur5_description)/urdf/common/2f_coupling.urdf.xacro"/>
<xacro:coupling_robotiq_2f prefix="" parent="tool0">
    <origin xyz="0 0 -0.0045" rpy="0 0 -1.5708"/>
</xacro:coupling_robotiq_2f>

<!-- GRIPPER -->
<xacro:include filename="$(find robotiq_85_description)/urdf/robotiq_85_gripper.urdf.xacro"/>
<xacro:robotiq_85_gripper prefix="" parent="coupling_gripper_fix">
    <origin xyz="0 0 0" rpy="0 -1.5708 0"/>
</xacro:robotiq_85_gripper>

<link name="tcp"/>

<joint name="tcp_to_gripper" type="fixed">
    <parent link="tool0"/>
    <child link="tcp"/>
    <origin xyz="0 0 0.19" rpy="0 0 0"/>
</joint>

<!-- GAZEBO -->
<gazebo>
        <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
            <robotNamespace>/</robotNamespace>
            <robotSimType>gazebo_ros_control/DefaultRobotHWSim</robotSimType>
        </plugin>
</gazebo>

</robot>

I then used moveit to create custom configuration and whenever I try to launch the gazebo simulation (gazebo.launch) I get the following error:

Failed to build tree: parent link [tool0] of joint [tcp_to_gripper] not found.  This is not valid according to the URDF spec. Every link you refer to from a joint needs to be explicitly defined in the robot description. To fix this problem you can either remove this joint [tcp_to_gripper] from your urdf file, or add "<link name="tool0" />" to your urdf file.

I tried removing the tcp link and rerunning the configuration, but again I end up with another error connecting to the robot itself:

Failed to build tree: child link [base_link] of joint [world_to_robot] not found.

In order to debug this problem, I checked my xacro file with the check_urdf command and the result looks fine

 name is: ur5_2f85
---------- Successfully Parsed XML ---------------
root Link: world has 1 child(ren)
    child(1):  base_link
        child(1):  base
        child(2):  shoulder_link
            child(1):  upper_arm_link
                child(1):  forearm_link
                    child(1):  wrist_1_link
                        child(1):  wrist_2_link
                            child(1):  wrist_3_link
                                child(1):  ee_link
                                child(2):  tool0
                                    child(1):  2f_coupling
                                        child(1):  coupling_gripper_fix
                                            child(1):  gripper_base_link
                                                child(1):  gripper_finger1_inner_knuckle_link
                                                    child(1):  gripper_finger1_finger_tip_link
                                                child(2):  gripper_finger1_knuckle_link
                                                    child(1):  gripper_finger1_finger_link
                                                child(3):  gripper_finger2_inner_knuckle_link
                                                    child(1):  gripper_finger2_finger_tip_link
                                                child(4):  gripper_finger2_knuckle_link
                                                    child(1):  gripper_finger2_finger_link
                                    child(2):  tcp

Finally, I ran rviz and the robot is correctly visualized and fully controllable

Any leads on how to ... (more)