Make sure LSP is ready before adding folder

Make sure we're not racing with LSP startup

Author: award999

src/sourcekit-lsp/LanguageClientToolchainCoordinator.ts

@@ -62,7 +62,10 @@ export class LanguageClientToolchainCoordinator implements vscode.Disposable {
             case FolderOperation.add: {
                 const client = await this.create(folder, singleServer, languageClientFactory);
                 await (singleServer
-                    ? client.addFolder(folder)
+                    ? client.useLanguageClient(async () => {
+                          // Just want to wait for client to be ready
+                          await client.addFolder(folder);
+                      })
                     : client.setLanguageClientFolder(folder));
                 break;
             }