**Steps to replicate the issue**:
# Run the example `pwb.py -v category_graph -from` as provided by the category_graph.py docstring.
**What happens?**:
# Notice that a `TypeError` exception occurs with message NoneType is not subscriptable" on line 93.
**Full Traceback:**
```
Exception has occurred: TypeError
'NoneType' object is not subscriptable
File "C:\git\core\scripts\category_graph.py", line 93, in __init__
self.dot = pydot.graph_from_dot_data(f'digraph {style}')[0]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
File "C:\git\core\scripts\category_graph.py", line 208, in <module>
bot = CategoryGraphBot(ap, args)
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\git\core\pywikibot\scripts\wrapper.py", line 147, in run_python_file
exec(compile(source, filename, 'exec', dont_inherit=True),
File "C:\git\core\pywikibot\scripts\wrapper.py", line 484, in execute
run_python_file(filename, script_args, module)
File "C:\git\core\pywikibot\scripts\wrapper.py", line 497, in main
if not execute():
^^^^^^^^^
File "C:\git\core\pywikibot\scripts\wrapper.py", line 513, in <module>
main()
File "C:\git\core\pwb.py", line 35, in main
runpy.run_path(str(path), run_name='__main__')
File "C:\git\core\pwb.py", line 39, in <module>
sys.exit(main())
^^^^^^
TypeError: 'NoneType' object is not subscriptable
```
**What should have happened instead?**:
- `pwb.py -v category_graph -from` should run without an exception.
**Software version**:
- pywikibot core master, commit 7bb835150a807463a0db018d3c023a735ce15df1
**Notes:**
The DOT-string is missing exterior curly braces. See [[ https://pypi.org/project/pydot/ | pydot ]] Input -> 2.
**Proposed Solution:**
Wrap the DOT-string in category_graph.py in curly braces.