BUG: app.builder can be None

To address:

```
Running Sphinx v5.0.2
making output directory... done

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/itkwidgets/envs/latest/lib/python3.7/site-packages/sphinx/cmd/build.py", line 275, in build_main
    args.tags, args.verbosity, args.jobs, args.keep_going)
  File "/home/docs/checkouts/readthedocs.org/user_builds/itkwidgets/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 237, in __init__
    self.config.setup(self)
  File "/home/docs/checkouts/readthedocs.org/user_builds/itkwidgets/checkouts/latest/docs/conf.py", line 79, in setup
    if RTD and app.builder.format == "html":
AttributeError: 'NoneType' object has no attribute 'format'
```

Author: thewtex

docs/conf.py

@@ -70,11 +70,12 @@
         '_output']
 
 def jupyterlite_build(app: Sphinx, error):
+    if error is not None and app.builder and app.builder.format == "html":
     subprocess.check_call(['jupyter', 'lite', 'build', '--config',
         str(jupyterlite_config)], cwd=str(here))
 
 def setup(app):
     # For local builds, run jupyter lite build manually
     # $ jupyter lite serve --config ./jupyterlite_config.json
-    if RTD and app.builder.format == "html":
+    if RTD:
         app.connect("config-inited", jupyterlite_build)