tf id frame odom_combined and base_footprint does not exist error

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Hi, how are you doing guys? Hope you are doing great.

I'm writing this message because I'm having problems with the pr2_controller tutorial: "Using the base controller with odometry and transform information", even though that the first time I ran it it worked fine. Let me explain everything I've done to see if anybody can find what is wrong.

The first time I ran it it gave me this error:

drive_base_tutorial$ bin/odometry_base terminate called after throwing an instance of 'tf::LookupException' what(): Frame id /base_footprint does not exist! When trying to transform between /odom_combined and /base_footprint. Aborted

After looking a little on the internet (I after realized that this was explained on the tutorial too) I found a girl with the exact same problem and somebody told her to use:

bin/odometry_base cmd_vel:=base_controller/command

I did it and it worked. I tried to use the same instruction on the same terminal to move the robot again, but the terminal froze. I dismissed this incident believing that I couldn't use the same instruction on the same terminal two times. I closed the terminal, ended the simulation, turned off the computer and went to lunch. I came back and tried to run it again and now everything is screwed. I try to run the simulation again and I keep getting:

drive_base_tutorial$  bin/odometry_base cmd_vel:=base_controller/command terminate called after throwing an instance of 'tf::LookupException'  what():  Frame id /base_footprint does not exist! When trying to transform between /odom_combined and /base_footprint. Aborted

or even

drive_base_tutorial$  bin/odometry_base cmd_vel:=base_controller/command terminate called after throwing an instance of 'tf::LookupException' what():  Frame id /odom_combined does not exist! When trying to transform between /odom_combined and /base_footprint. Aborted

I've checked

rosrun tf tf_monitor base_footprint odom_combined

and it just keeps drawing points without telling me nothing. I've also tried:

rosrun tf tf_echo base_footprint odom_combined

and again it tells me that one of the frames do not exist. I erased the package drive_base_tutorial and did everything from the beginning and nothing. My questions are: What is this frame odom_combined? I know that base_footprint is created on the pr2 URDF, but I can't find this odom_combined anywhere. Is it in any controller of the pr2? which one?.

Can't I change it for other existing frame? on the turtlesim/tf tutorials they use a "world" frame. Is there a world frame for gazebo? if there is, how is it called? I've tried changing odom_combined for world but this frame doesn't exist either.

I've found tutorials that teaches you how to add a new frame, but is a new child frame, can I create a world frame from scratch? how?

Please help me. Also consider that I'm very new to programming so please have patience. Sorry it was such a long history, but I do really want to understand what is going on.

Thanks for you time, patience and for any help you can give me.

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