Use conditional access.

Author: JoeRobich

src/lsptoolshost/services/projectContextService.ts

@@ -46,7 +46,7 @@ export class ProjectContextService {
         const uri = textEditor.document.uri;
 
         // If we have an open request, cancel it.
-        this._source.cancel();
+        this._source?.cancel();
         this._source = new vscode.CancellationTokenSource();
 
         try {