Problems with ur3 Simulation

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Hello,

Im pretty new to ros and im trying to get a simulation of a ur3 to work. Ive been following the instructions on the universal_robot Git but i seem to get multiple error messages.

When i clone the Git to my catkin workspace and run all the rosrun commands as instructed

roslaunch ur_description ur3_upload.launch 
roslaunch ur3_moveit_config ur3_moveit_planning_execution.launch sim:=true
roslaunch ur3_moveit_config moveit_rviz.launch config:=true

the messages i get are:

Skipping virtual joint 'fixed_base' because its child frame 'base_link' does not match the URDF frame 'world'

,

[ WARN] [1532077114.269386647]: Waiting for /follow_joint_trajectory to come up
[ WARN] [1532077120.271582767]: Waiting for /follow_joint_trajectory to come up
[ERROR] [1532077126.272691382]: Action client not connected: /follow_joint_trajectory

and in RViz i get

Global Status: Warn 
    No tf data. Actual error: Fixed Frame [world] does not exist

I could get rid of the first problem by following this suggestion but renaming all the "world" links doesn't get me any further in solving my "missing fixed world frame" problem. Any suggestions would be appreciated.