Smalltalk has a great debugger. You can bring it up whenever you have an error, check all the variables and data structures (even bring up inspectors), then actually modify the code and go on.
Whenever you get an error, you'll get a window like this:
Use the middle mouse button to bring up the debugger.
The top pane shows you where the code has been. Click on any of those lines to see different settings below. The bottommost panes show you the instance variables of self and the local variables in the code. You can inspect any of these. The middle pane is the actual source that was executing. You can change this and re-accept it. This will change your code as if you had done it from the System Browser.