!_.isUndefied(null) throws TypeError: Cannot read property '$ref' of undefined

[sanjulr]

1. What version of the module is the issue happening on? Does the issue happen on latest version?
   Issue is present in the latest version.

2. What platform and Node.js version? (For example Node.js 0.12 on Mac OS X)
   12.13.0 and Mac OS X

3. Sample source code or steps to reproduce
   openapi-validator-middleware uses this library and when initiating that middleware with a json spec, it throws TypeError: Cannot read property '$ref' of undefined.

The fix is pretty simple. In this library's checkLocalCircular and derefSchema function, change !_.isUndefined(null) to !_.isUndefined(node)

[tovbinm]

We discovered the same problem. Please merge and release the fix.