`finally` is not a function error hit with `_getServerWorkingDir`

TypeError this._getServerWorkingDir(...).then(...).finally is not a function 
    extension.js:1:395787 d.createMessageTransports
    extension.js:1:226888 d.createConnection
    extension.js:1:217342 d.start
    extension.js:1:130234 _

https://github.com/microsoft/vscode-languageserver-node/blob/df56e720c01c6e2d7873733807418f6ce33187ad/client/src/node/main.ts#L573-L589

I see this error in the telemetry of our VS Code extension. If I try to search the error on the Internet, I can find a single StackOverflow post talking about the same thing. I have looked at the code and tried writing a small JavaScript program but can't quite figure out in what case this might happen. We are on 9.0.1 of vscode-languageclient.

Start with client/src/node/main.ts lines 573-589 and inspect the _getServerWorkingDir promise chain, then compare it with the reported vscode-languageclient 9.0.1 environment. Reproduce the error in the affected runtime or extension context and establish which condition yields a value without finally; done means the cause is confirmed and the failure is prevented or covered by an appropriate regression check.