Failed to run openapi-cop TypeError: Cannot read property 'constructor' of null

Starting revision 5cb4e9f988cc7150374db3b3dad383281eaf7c8d (from PR #404) in Docker (compose) I get the below output.

Note also that it `exited with code 0` which is weird considering it didn't work.

# Environment

```
DEFAULT_FORBID_ADDITIONAL_PROPERTIES=true
VERBOSE=true
TARGET=http://xxxxxx:5058
FILE=/api.yaml
```

# `api.yaml`

Proprietary, sorry, can't post it.

# Output

Note how everything here is `.js` and the line numbers not matching the `.ts` source code.

```
pfcapi-openapi-cop-1 | Validating against api.yaml ("PFC API", version: 1.0.0)
pfcapi-openapi-cop-1 | Additional properties will be forbidden by default. Existing `additionalProperties` settings in the OpenAPI document will NOT be overwritten.
pfcapi-openapi-cop-1 | Failed to run openapi-cop TypeError: Cannot read property 'constructor' of null
pfcapi-openapi-cop-1 | at mapWalkObject (/openapi-cop-docker/src/util.js:209:19)
pfcapi-openapi-cop-1 | at mapWalkObject (/openapi-cop-docker/src/util.js:210:28)
pfcapi-openapi-cop-1 | at mapWalkObject (/openapi-cop-docker/src/util.js:210:28)
pfcapi-openapi-cop-1 | at mapWalkObject (/openapi-cop-docker/src/util.js:210:28)
pfcapi-openapi-cop-1 | at mapWalkObject (/openapi-cop-docker/src/util.js:210:28)
pfcapi-openapi-cop-1 | at Object.mapWalkObject (/openapi-cop-docker/src/util.js:210:28)
pfcapi-openapi-cop-1 | at prepareApiDocument (/openapi-cop-docker/src/app.js:184:21)
pfcapi-openapi-cop-1 | at process._tickCallback (internal/process/next_tick.js:68:7)
pfcapi-openapi-cop-1 | at Function.Module.runMain (internal/modules/cjs/loader.js:834:11)
pfcapi-openapi-cop-1 | at startup (internal/bootstrap/node.js:283:19)
pfcapi-openapi-cop-1 | at bootstrapNodeJSCore (internal/bootstrap/node.js:623:3)
pfcapi-openapi-cop-1 exited with code 0
```