Creating Histogram from Python

Hello, I am trying to create a histogram using Root from Python. However, I am getting this warning:
Execution of your code was aborted.
ROOT_prompt_2:1:1: warning: null passed to a callee that requires a non-null argument [-Wnonnull]

There is alot of other outputs above the warning, but I am not sure if it meant much. Do you know how I can fix this warning.

The variable histo is invalid: the pointer is NULL.