under these circumstances, I would go with option A. In this case you are doing your input validation and then preventing execution of the rest of the code if the input is not valid (not editable). This keeps the entire body of the function out of a big if/else statement and makes it more readable.
However, I would also consider raising an exception rather than retuning a null - that is assuming that passing in a non-editable object into an "edit" function isn't a normal occurrence.
