Fix getBranchPoints accessor (#8695)

Regression from #8626. The branchpoints need to be a flat list so that
the length can be compared to 0.

- https://___.webknossos.xyz

- create a new tree with node
- press J

- fixes
https://discuss.webknossos.org/t/absence-of-jumps-in-annotations-is-not-detected/1930/2

------
(Please delete unneeded items, merge only when none are left open)
- [x] Updated
[changelog](../blob/master/CHANGELOG.unreleased.md#unreleased)

Author: philippotto

CHANGELOG.unreleased.md

@@ -15,6 +15,7 @@ For upgrade instructions, please check the [migration guide](MIGRATIONS.released
 ### Changed
 
 ### Fixed
+- Fixed a regression that led to incorrect behavior when trying to jump to the last branchpoint even though no branchpoint existed. [#8695](https://github.com/scalableminds/webknossos/pull/8695)
 
 ### Removed
 

frontend/javascripts/viewer/model/accessors/skeletontracing_accessor.ts

@@ -276,13 +276,16 @@ export function getMaxNodeId(skeletonTracing: SkeletonTracing): number | null {
   );
 }
 
-export function getBranchPoints(annotation: StoreAnnotation): IteratorObject<BranchPoint[]> | null {
+export function getBranchPoints(annotation: StoreAnnotation): BranchPoint[] | null {
   const skeletonTracing = getSkeletonTracing(annotation);
   if (skeletonTracing == null) {
     return null;
   }
 
-  return skeletonTracing.trees.values().map((tree) => tree.branchPoints);
+  return skeletonTracing.trees
+    .values()
+    .flatMap((tree) => tree.branchPoints)
+    .toArray();
 }
 
 export function getFlatTreeGroups(

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

@@ -126,7 +126,7 @@ function* watchBranchPointDeletion(): Saga<void> {
 
     if (deleteBranchpointAction) {
       const hasBranchPoints = yield* select(
-        (state: WebknossosState) => (getBranchPoints(state.annotation)?.toArray() ?? []).length > 0,
+        (state: WebknossosState) => (getBranchPoints(state.annotation) ?? []).length > 0,
       );
 
       if (hasBranchPoints) {