src/vs/workbench/browser/parts/editor/editorPart.ts/EditorPart/assertGroupView
class EditorPart:	private assertGroupView(group: IEditorGroupView | GroupIdentifier): IEditorGroupView {
		let groupView: IEditorGroupView | undefined;
		if (typeof group === 'number') {
			groupView = this.getGroup(group);
		} else {
			groupView = group;
		}

		if (!groupView) {
			throw new Error('Invalid editor group provided!');
		}

		return groupView;
	}