Extension crashes if `flow.trace.server` is not a string

[jvilk-stripe]

Steps to reproduce

1. Add flow.trace.server: null to your VS Code settings.
2. Restart VS Code.
3. Open a JS file. Flow will fail to start. The log shows a type error in fromString.

Expected result

Flow ignores the invalid setting.

The issue appears to be in the LanguageClient library. From the minified source, I cannot tell if it has been fixed upstream:

https://github.com/microsoft/vscode-languageserver-node/blob/084c035af6aeb2999df7d26eb69f3b34acae9ca8/client/src/common/client.ts#L3339