Fix: resolve undefined variable and early termination in aveElecStatPot.py

[DanielZhao0432]

Linked Issue

Fixes #7559

Description

This PR addresses two logical issues in the aveElecStatPot.py script that could cause it to crash or exit prematurely when searching for ElecStaticPot.cube:

1. Premature Termination (Early sys.exit):
   Previously, the script would immediately exit (sys.exit()) if the first matching OUT.* subdirectory scanned did not contain ElecStaticPot.cube. This prevented the script from checking subsequent OUT.* directories that might actually contain the target file.

2. NameError (Undefined input_file):
   If no ElecStaticPot.cube was present in the root directory, and no OUT.* subdirectories existed, the nested search loop was never executed. Consequently, input_file was never defined, resulting in a NameError when the script reached the with open(input_file, 'r') block.

Changes Made

• Initialized input_file, output_file, and output_png to None to prevent NameError.
• Updated the directory scanning logic so that the script continues searching through all candidate OUT.* directories rather than exiting immediately on the first miss.
• Added a unified check after the search block: if the file cannot be found anywhere, the script now exits cleanly with an informative error message.