TypeError: Cannot read property 'getText' of null. Fixes Microsoft/monaco-editor#354

src/cssWorker.ts

@@ -43,10 +43,13 @@ export class CSSWorker {
 
     doValidation(uri: string): Promise<ls.Diagnostic[]> {
 		let document = this._getTextDocument(uri);
+		if (document) {
 			let stylesheet = this._languageService.parseStylesheet(document);
 			let diagnostics = this._languageService.doValidation(document, stylesheet);
 			return Promise.as(diagnostics)
 		}
+		return Promise.as([]);
+	}
     doComplete(uri: string, position: ls.Position): Promise<ls.CompletionList> {
 		let document = this._getTextDocument(uri);
 		let stylesheet = this._languageService.parseStylesheet(document);