FCPX reader fails reading clip with solid color generator

I got the following error when reading part of the .fcpxml file.

**'NoneType' object has no attribute 'find'**

```
Traceback (most recent call last):
File "/data/papepipe/pg_production_lib/src/lib/production/edit/otio/otio_checking.py", line 43, in
info(xml)
File "/data/papepipe/pg_production_lib/src/lib/production/edit/otio/otio_checking.py", line 20, in info
tl = otio.adapters.read_from_file(xml_path)
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio/adapters/__init__.py", line 144, in read_from_file
**adapter_argument_map
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio/adapters/adapter.py", line 134, in read_from_file
**adapter_argument_map
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio/plugins/python_plugin.py", line 174, in _execute_function
return (getattr(self.module(), func_name)(**kwargs))
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio_contrib/adapters/fcpx_xml.py", line 1168, in read_from_string
return FcpxXml(input_str).to_otio()
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio_contrib/adapters/fcpx_xml.py", line 795, in to_otio
return self._from_library()
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio_contrib/adapters/fcpx_xml.py", line 806, in _from_library
return self._from_event(self.fcpx_xml.find("./library/event"))
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio_contrib/adapters/fcpx_xml.py", line 813, in _from_event
container.append(self._from_project(project))
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio_contrib/adapters/fcpx_xml.py", line 819, in _from_project
project_element.find("./sequence", {})
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio_contrib/adapters/fcpx_xml.py", line 853, in _squence_to_stack
sequence_element.get("format")
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio_contrib/adapters/fcpx_xml.py", line 900, in _build_composable
self._format_id_for_clip(element, default_format)
File "/l/packages/pg/third_party/plugin/python_lib/0.0.11/platform-linux/lib/site-packages/opentimelineio_contrib/adapters/fcpx_xml.py", line 1020, in _format_id_for_clip
clip.get("ref")
AttributeError: 'NoneType' object has no attribute 'find'
```
## To Reproduce

1. Operating System: CentOS 7
2. Python version: Pythoin 2.7.18
3. OpenTimelineIO release version or commit hash: OpenTimelineIO==0.13.0

Reproduce the failure with the reported FCPXML and inspect opentimelineio_contrib/adapters/fcpx_xml.py, especially _from_project, _squence_to_stack, _build_composable, and _format_id_for_clip. Confirm how a solid color generator is represented, then verify that reading the file completes without the NoneType exception.

Written by the indexing model from the issue text.