Problem with attaching a robotiq 2f gripper to my kuka lwr [closed]

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Hello, I want to attach a gripper to my kuka lwr. I found the robotiq 2f gripper to fit quite good for my requirements ( https://github.com/Danfoa/robotiq_2fi... ). But I have some problems with attaching the gripper to my robot. I have a urdf.xarco file where I include all my separate parts

<?xml version="1.0"?>
  <robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="single_lwr_robot">

  <!-- Include all models -->
  <xacro:include filename="$(find lwr_description)/model/kuka_lwr.urdf.xacro"/>
  <xacro:include filename="$(find robotiq_2f_140_gripper_visualization)/urdf/robotiq_arg2f_140_model_macro.xacro" />

  <xacro:kuka_lwr name="lwr">
  <origin xyz="0 0 0" rpy="0 0 0"/>
  </xacro:kuka_lwr> 

  <xacro:robotiq_arg2f_140 />
</robot>

The kuka robot alone works fine, I was also able to add a box at the top of the robot. Therefore I made a new joint on the last link of the robot and attached the box to this link. When I do the same with the robotiq gripper, it doesn't work. Here are is my changed robotiq_arg2f.xarco file:

<?xml version="1.0"?>
<robot xmlns:xacro="http://ros.org/wiki/xacro">
  <xacro:macro name="robotiq_arg2f_base_link" params="prefix">

  <joint name="tap_joint" type="fixed">
    <origin xyz="0 0 0.01" rpy="0 0 0"/>
    <parent link="lwr_7_link"/>
    <child link="${prefix}robotiq_arg2f_base_link"/>
  </joint>

    <link name="${prefix}robotiq_arg2f_base_link">
      <inertial>
        <origin xyz="8.625E-08 -4.6583E-06 0.03145" rpy="0 0 0" />
        <mass value="0.22652" />
        <inertia ixx="0.00020005" ixy="-4.2442E-10" ixz="-2.9069E-10" iyy="0.00017832" iyz="-3.4402E-08" izz="0.00013478" />
      </inertial>
      <visual>
        <origin xyz="0 0 0" rpy="0 0 0" />
        <geometry>
          <mesh filename="package://robotiq_2f_140_gripper_visualization/meshes/robotiq_arg2f_base_link_fine.stl" />
        </geometry>
        <material name="">
          <color rgba="0.1 0.1 0.1 1" />
        </material>
      </visual>
      <collision>
        <origin xyz="0 0 0" rpy="0 0 ${pi / 2}" />
        <geometry>
          <mesh filename="package://robotiq_2f_140_gripper_visualization/meshes/robotiq_arg2f_base_link_coarse.stl" />
        </geometry>
      </collision>
    </link>
  </xacro:macro>

  <xacro:macro name="finger_joints" params="prefix fingerprefix reflect">
    <xacro:outer_finger_joint prefix="${prefix}" fingerprefix="${fingerprefix}"/>
    <xacro:inner_knuckle_joint prefix="${prefix}" fingerprefix="${fingerprefix}" reflect="${reflect}"/>
    <xacro:inner_finger_joint prefix="${prefix}" fingerprefix="${fingerprefix}"/>
  </xacro:macro>

  <xacro:macro name="finger_links" params="prefix fingerprefix stroke">
    <xacro:outer_knuckle prefix="${prefix}" fingerprefix="${fingerprefix}" stroke="${stroke}"/>
    <xacro:outer_finger prefix="${prefix}" fingerprefix="${fingerprefix}" stroke="${stroke}"/>
    <xacro:inner_finger prefix="${prefix}" fingerprefix="${fingerprefix}" stroke="${stroke}"/>
    <xacro:inner_knuckle prefix="${prefix}" fingerprefix="${fingerprefix}" stroke="${stroke}"/>
  </xacro:macro>
</robot>

The tap_joint is the part I've added. But now comes the problem. Gazebo and RViz are starting up but showing strange shapes of the gripper (RViz only shows the ground part without the fingers and in Gazebo the gripper is kinda falling apart). RVIz shows the following errors regarding the fingers: No transform from [left_inner_finger] to [world]. What am I doing wrong?