VSCode API TreeView not getting children of elements

In providing a treeview to VSCode, it seems to get all the parent elements but fail to get the children of these parent elements, I've added a debug point into it and saw that it was only ever called with undefined, is there something improper with my implementation?

export class GalleryTreeItem extends vscode.TreeItem { constructor( private extensionUri: vscode.Uri, public collapsibleState: vscode.TreeItemCollapsibleState, public readonly name: string, public readonly project?: Project ) { super(name, collapsibleState); if (this.project) { this.contextValue = "gallery"; this.description = `v${this.project.config.userContentVersion}`; this.tooltip = this.project.config.repositoryUrl; this.command = { title: "Plywood Gallery: Open a gallery webview.", command: "plywood-gallery.Open", arguments: [this.label], }; this.iconPath = vscode.Uri.joinPath( this.extensionUri, "assets/photo-gallery.png" ); } else { this.contextValue = "chapter"; } } getChapters() { if (this.project) { return Object.keys(this.project.parameters).map( (name) => new GalleryTreeItem( this.extensionUri, vscode.TreeItemCollapsibleState.Collapsed, name ) ); } else { return []; } } } export class InstalledGalleriesExplorerProvider implements vscode.TreeDataProvider<GalleryTreeItem> { constructor(private extensionUri: vscode.Uri) {} private _onDidChangeTreeData: vscode.EventEmitter< GalleryTreeItem | undefined | void > = new vscode.EventEmitter<GalleryTreeItem | undefined | void>(); readonly onDidChangeTreeData: vscode.Event< GalleryTreeItem | undefined | void > = this._onDidChangeTreeData.event; getTreeItem(element: GalleryTreeItem): vscode.TreeItem { return element; } refresh(): void { this._onDidChangeTreeData.fire(); } async getChildren(element?: GalleryTreeItem): Promise<GalleryTreeItem[]> { if (element) { return Promise.resolve(element.getChapters()); } else { return getLocalProjects(this.extensionUri).then((projects) => projects.map( (prj) => new GalleryTreeItem( this.extensionUri, vscode.TreeItemCollapsibleState.None, prj.config.projectName, prj ) ) ); } } }