'Kinematics' object has no attribute 'chain' in PyKDL

I wonder is there something wrong with the version of PyKDL, or just the urdf reader fail to extract the file. When I run `src/kdl_kinematic.py`, error occured,
```
File "/xxx/mujoco-learning/src/kdl_kinematic.py", line 33, in createSolver
    self.fk_solver = PyKDL.ChainFkSolverPos_recursive(self.chain)
AttributeError: 'Kinematics' object has no attribute 'chain'
```