Fix error when requesting mappings for volume tracings without fallback (#8710)

### URL of deployed dev instance (used for testing):
- https://___.webknossos.xyz

### Steps to test:
- create a new annotation with a volume tracing that does not have a
fallback layer (e.g., simply create a new empty volume layer within
another annotation to get a blank one)
- hover over the proofreading tool --> should not crash

### Issues:
- fixes https://scm.slack.com/archives/C02H5T8Q08P/p1750692063338429

------
(Please delete unneeded items, merge only when none are left open)
- [x] Added changelog entry (create a `$PR_NUMBER.md` file in
`unreleased_changes` or use `./tools/create-changelog-entry.py`)
- [ ] Added migration guide entry if applicable (edit the same file as
for the changelog)
- [ ] Updated [documentation](../blob/master/docs) if applicable
- [ ] Adapted [wk-libs python
client](https://github.com/scalableminds/webknossos-libs/tree/master/webknossos/webknossos/client)
if relevant API parts change
- [ ] Removed dev-only changes like prints and application.conf edits
- [ ] Considered [common edge
cases](../blob/master/.github/common_edge_cases.md)
- [ ] Needs datastore update after deployment

Author: philippotto

frontend/javascripts/viewer/model/sagas/mapping_saga.ts

@@ -311,17 +311,27 @@ function* loadLayerMappings(layerName: string, updateInStore: boolean): Saga<[st
     return [layerInfo.mappings, layerInfo.agglomerates];
   }
 
-  const params = [
-    dataset.dataStore.url,
-    dataset, // If there is a fallbackLayer, request mappings for that instead of the tracing segmentation layer
-    "fallbackLayer" in layerInfo && layerInfo.fallbackLayer != null
-      ? layerInfo.fallbackLayer
-      : layerInfo.name,
-  ] as const;
-  const [jsonMappings, serverHdf5Mappings] = yield* all([
-    call(getMappingsForDatasetLayer, ...params),
-    call(getAgglomeratesForDatasetLayer, ...params),
-  ]);
+  let jsonMappings: string[];
+  let serverHdf5Mappings: string[];
+
+  if (layerInfo.tracingId != null && layerInfo.fallbackLayer == null) {
+    // The layer is a volume tracing without fallback layer. No mappings
+    // exist for these kind of layers and should not be requested from the server.
+    jsonMappings = [];
+    serverHdf5Mappings = [];
+  } else {
+    const params = [
+      dataset.dataStore.url,
+      dataset, // If there is a fallbackLayer, request mappings for that instead of the tracing segmentation layer
+      "fallbackLayer" in layerInfo && layerInfo.fallbackLayer != null
+        ? layerInfo.fallbackLayer
+        : layerInfo.name,
+    ] as const;
+    [jsonMappings, serverHdf5Mappings] = yield* all([
+      call(getMappingsForDatasetLayer, ...params),
+      call(getAgglomeratesForDatasetLayer, ...params),
+    ]);
+  }
 
   if (updateInStore) {
     yield* put(setLayerMappingsAction(layerName, jsonMappings, serverHdf5Mappings));

tools/create-changelog-entry.py

@@ -5,16 +5,16 @@
 import sys
 
 TEMPLATE = """### Added
-- This is an example. You may delete a section if it would become empty.
+- Added xyz... This is an example. You may delete a section if it would become empty.
 
 ### Changed
-- This is an example. You may delete a section if it would become empty.
+- Changed xyz... This is an example. You may delete a section if it would become empty.
 
 ### Fixed
-- This is an example. You may delete a section if it would become empty.
+- Fixed xyz... This is an example. You may delete a section if it would become empty.
 
 ### Removed
-- This is an example. You may delete a section if it would become empty.
+- Removed xyz... This is an example. You may delete a section if it would become empty.
 
 ### Breaking Changes
 - This is an example. You may delete a section if it would become empty.

unreleased_changes/8710.md

@@ -0,0 +1,2 @@
+### Fixed
+- Fixed that WEBKNOSSOS could crash when mappings were requested for a volume layer that is not based on another segmentation layer.