Fix microsoft/vscode#63805

src/services/selectorPrinting.ts

@@ -357,9 +357,13 @@ function selectorToSpecificityMarkedString(node: nodes.Node): MarkedString {
 
 export function selectorToMarkedString(node: nodes.Selector): MarkedString[] {
 	const root = selectorToElement(node);
-	const markedStrings = new MarkedStringPrinter('"').print(root);
-	markedStrings.push(selectorToSpecificityMarkedString(node));
-	return markedStrings;
+	if (root) {
+		const markedStrings = new MarkedStringPrinter('"').print(root);
+		markedStrings.push(selectorToSpecificityMarkedString(node));
+		return markedStrings;
+	} else {
+		return [];
+	}
 }
 
 export function simpleSelectorToMarkedString(node: nodes.SimpleSelector): MarkedString[] {
@@ -438,7 +442,7 @@ function isNewSelectorContext(node: nodes.Node): boolean {
 	return false;
 }
 
-export function selectorToElement(node: nodes.Selector): Element {
+export function selectorToElement(node: nodes.Selector): Element | null {
 	if (node.matches('@at-root')) {
 		return null;
 	}

src/test/css/hover.test.ts

@@ -0,0 +1,36 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+import * as assert from 'assert';
+import { TextDocument, FoldingRange, FoldingRangeKind, Hover } from 'vscode-languageserver-types';
+import { CSSHover } from '../../services/cssHover';
+import { SCSSParser } from '../../parser/scssParser';
+
+function assertSCSSHover(value: string, expected: Hover): void {
+	const languageId = 'scss'
+
+	let offset = value.indexOf('|');
+	value = value.substr(0, offset) + value.substr(offset + 1);
+
+	const hover = new CSSHover()
+	const document = TextDocument.create(`test://foo/bar.${languageId}`, languageId, 1, value);
+	const hoverResult = hover.doHover(document, document.positionAt(offset), new SCSSParser().parseStylesheet(document));
+
+	if (hoverResult.range && expected.range) {
+		assert.equal(hoverResult.range, expected.range);
+	}
+	assert.deepEqual(hoverResult.contents, expected.contents);
+}
+
+suite('SCSS Hover', () => {
+	test('@at-root', () => {
+
+		assertSCSSHover('.test { @|at-root { }', {
+			contents: []
+		})
+	});
+});