another try, #41408

src/vs/workbench/api/node/extHost.protocol.ts

@@ -72,7 +72,7 @@ export interface IWorkspaceData {
 	id: string;
 	name: string;
 	folders: { uri: UriComponents, name: string, index: number }[];
-	configuration: UriComponents;
+	configuration?: UriComponents;
 }
 
 export interface IInitData {

src/vs/workbench/api/node/extHostExtensionService.ts

@@ -109,7 +109,7 @@ class ExtensionStoragePath {
 				join(storagePath, 'meta.json'),
 				JSON.stringify({
 					id: this._workspace.id,
-					configuration: URI.revive(this._workspace.configuration).toString(),
+					configuration: this._workspace.configuration && URI.revive(this._workspace.configuration).toString(),
 					name: this._workspace.name
 				}, undefined, 2)
 			);