I have a DF where some values have a prefix and I want to make a loop to remove it.
The DF looks like this:
[![enter image description here](https://i.stack.imgur.com/kKylC.png)](https://i.stack.imgur.com/kKylC.png)

I want to remove the "null-" and just leave the numbers.
I tried that, but I got and error message:
[![enter image description here](https://i.stack.imgur.com/ruzUM.png)](https://i.stack.imgur.com/ruzUM.png)