CSS Language server error

Issue Type: Bug

I copied and pasted some scss, and the panel opened with this error

Or, as text:

[Error - 8:51:41 AM] Error while computing hover for file:///Users/st016lo/github/ng-designsystem/client/src/sass/designsystem/_layout.scss: Cannot read property 'addChild' of undefined
TypeError: Cannot read property 'addChild' of undefined
    at e.processSelector (/Users/st016lo/Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/extensions/css-language-features/server/dist/cssServerMain.js:1:565456)
    at /Users/st016lo/Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/extensions/css-language-features/server/dist/cssServerMain.js:1:564409
    at _n (/Users/st016lo/Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/extensions/css-language-features/server/dist/cssServerMain.js:1:564430)
    at e.doHover (/Users/st016lo/Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/extensions/css-language-features/server/dist/cssServerMain.js:1:566036)
    at /Users/st016lo/Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/extensions/css-language-features/server/dist/cssServerMain.js:1:66230
    at Immediate.<anonymous> (/Users/st016lo/Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/extensions/css-language-features/server/dist/cssServerMain.js:1:110959)
    at runCallback (timers.js:789:20)
    at tryOnImmediate (timers.js:751:5)
    at processImmediate [as _immediateCallback] (timers.js:722:5)

VS Code version: Code - Insiders 1.28.0-insider (ee6a21fa49bd1906bd1378c2f2714cd656409e5c, 2018-09-11T05:12:02.233Z) OS version: Darwin x64 17.7.0

System Info

|Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 x 2800)| |GPU Status|2d_canvas: enabled
checker_imaging: disabled_off
flash_3d: enabled
flash_stage3d: enabled
flash_stage3d_baseline: enabled
gpu_compositing: enabled
multiple_raster_threads: enabled_on
native_gpu_memory_buffers: enabled
rasterization: enabled
video_decode: enabled
video_encode: enabled
webgl: enabled
webgl2: enabled| |Load (avg)|2, 3, 3| |Memory (System)|16.00GB (0.23GB free)| |Process Argv|/Users/st016lo/Applications/Visual Studio Code - Insiders.app/Contents/MacOS/Electron -psn_0_36438718| |Screen Reader|no| |VM|0%|

Extensions (26)

Extension|Author (truncated)|Version ---|---|--- better-comments|aar|1.2.9 Bookmarks|ale|9.0.3 project-manager|ale|8.0.0 bracket-pair-colorizer|Coe|1.0.59 vscode-eslint|dba|1.6.0 tslint|eg2|1.0.38 vscode-great-icons|emm|2.1.41 prettier-vscode|esb|1.6.1 path-autocomplete|ion|1.12.0 svg|joc|0.1.2 ecdc|mit|0.12.0 HTMLHint|mka|0.5.0 theme-monokai-pro-vscode|mon|1.1.8 vscode-scss|mrm|0.6.2 vscode-stylefmt|mrm|2.5.0 debugger-for-chrome|msj|4.10.0 angular2-inline|nat|0.0.17 incrementor|nms|0.1.0 vscode-versionlens|pfl|0.21.1 quicktype|qui|12.0.42 code-settings-sync|Sha|3.1.2 stylelint|shi|0.43.0 rewrap|stk|1.9.1 sort-lines|Tyr|1.7.0 vscode-terminal-here|Tyr|0.1.1 change-case|wma|1.0.0 (2 theme extensions excluded)

microsoft / vscode

CSS Language server error #58433