Fix adding new layer via advanced dataset settings tab (#8550)

* fix adding new layer via advanced dataset settings tab

* add changelog entry

---------

Co-authored-by: Florian M <fm3@users.noreply.github.com>

Author: MichaelBuessemeyer

CHANGELOG.unreleased.md

@@ -32,6 +32,7 @@ For upgrade instructions, please check the [migration guide](MIGRATIONS.released
 - Fixed the alignment of the button that allows restricting floodfill operations to a bounding box. [#8388](https://github.com/scalableminds/webknossos/pull/8388) 
 - Fixed that it was possible to trigger the find largest segment id job on layers which are not stored as segmentation layers on the server. [#8503](https://github.com/scalableminds/webknossos/pull/8503)
 - Fixed resolution of legacy links having dataset names with `-`. [#8548](https://github.com/scalableminds/webknossos/pull/8548)
+- Fixed that adding a layer using the dataset settings' advanced tab would crash WEBKNOSSOS. Bug was introduced by [#8503](https://github.com/scalableminds/webknossos/pull/8503).  [#8550](https://github.com/scalableminds/webknossos/pull/8550)
 - Fixed a rare and subtle bug related to volume annotation and undo/redo. [#7506](https://github.com/scalableminds/webknossos/pull/7506)
 - Fixed a bug where segment statistics would sometimes be wrong in case of an on-disk segmentation fallback layer with segment index file. [#8460](https://github.com/scalableminds/webknossos/pull/8460)
 - Fixed a bug where sometimes outdated segment statistics would be displayed. [#8460](https://github.com/scalableminds/webknossos/pull/8460)

frontend/javascripts/dashboard/dataset/dataset_settings_data_tab.tsx

@@ -359,7 +359,7 @@ function SimpleLayerForm({
   form: FormInstance;
   dataset: APIDataset | null | undefined;
 }) {
-  const layerCategorySavedOnServer = dataset?.dataSource.dataLayers[index].category;
+  const layerCategorySavedOnServer = dataset?.dataSource.dataLayers[index]?.category;
   const isStoredAsSegmentationLayer = layerCategorySavedOnServer === "segmentation";
   const dataLayers = Form.useWatch(["dataSource", "dataLayers"]);
   const category = Form.useWatch(["dataSource", "dataLayers", index, "category"]);