Fix undo saga crash (#8868)

Noticed via airbrake. 
-
https://scm.airbrake.io/projects/95438/groups/4109902292512992645?filters=%7B%22order%22%3A%22last_notice%22%2C%22resolved%22%3A%22false%22%2C%22search%22%3A%22root%20saga%22%7D&tab=notice-detail
-
https://scm.airbrake.io/projects/95438/groups/4103951939158207307?filters=%7B%22order%22%3A%22last_notice%22%2C%22resolved%22%3A%22false%22%2C%22search%22%3A%22root%20saga%22%7D&tab=overview

The FE sometimes seems to fire update segment groups batch actions that
are empty, resulting in the undo saga crashing. I coded a little fix for
this.

> I added multiple safety guards against this bug:
> 1. The segments tab can no longer trigger an
`BatchUpdateGroupsAndSegmentsAction` with an empty actions array.
> 2. The undo saga is resilient against
`BatchUpdateGroupsAndSegmentsAction` actions with an empty payload
array.

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

### Steps to test:
- On master
- create an empty segment group
- on that group, change the color of all children via context menu on
that empty group -> watch wk crash
- on this branch, do the same -> the color changing is a no-op without
wk crashing.


### Issues:
- contributes to #8715 

------
(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`)

Author: MichaelBuessemeyer

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

@@ -409,13 +409,17 @@ export function* manageUndoStates(): Saga<never> {
           }
         }
       } else if (setSegmentGroups || batchUpdateGroupsAndSegments) {
-        if (setSegmentGroups?.calledFromUndoSaga) {
+        if (
+          setSegmentGroups?.calledFromUndoSaga ||
+          (setSegmentGroups == null && (batchUpdateGroupsAndSegments?.payload?.length ?? 0) <= 0)
+        ) {
           // Ignore this action as it was dispatched from within this saga.
+          // Or in case the BatchUpdateGroupsAndSegmentsAction does not have a payload and thus is a no-op.
           continue;
         }
         shouldClearRedoState = true;
         const layerName =
-          setSegmentGroups?.layerName || batchUpdateGroupsAndSegments?.payload[0].layerName;
+          setSegmentGroups?.layerName || batchUpdateGroupsAndSegments?.payload?.[0]?.layerName;
         if (layerName == null) {
           throw new Error("Could not find layer name for action.");
         }

frontend/javascripts/viewer/view/right-border-tabs/segments_tab/segments_view.tsx

@@ -286,7 +286,9 @@ const mapDispatchToProps = (dispatch: Dispatch<any>) => ({
     const actions = segmentIds.map((segmentId) =>
       updateSegmentAction(segmentId, segmentShape, layerName, undefined, createsNewUndoState),
     );
-    Store.dispatch(batchUpdateGroupsAndSegmentsAction(actions));
+    if (actions.length > 0) {
+      Store.dispatch(batchUpdateGroupsAndSegmentsAction(actions));
+    }
   },
 
   updateSegment(
@@ -1340,7 +1342,7 @@ class SegmentsView extends React.Component<Props, State> {
     if (visibleSegmentationLayer == null) return;
     const relevantSegments =
       groupId != null ? this.getSegmentsOfGroupRecursively(groupId) : this.getSelectedSegments();
-    if (relevantSegments == null) return;
+    if (relevantSegments == null || relevantSegments.length < 1) return;
 
     const actions = relevantSegments.map((segment) =>
       updateSegmentAction(segment.id, { color: color }, visibleSegmentationLayer.name),

unreleased_changes/8868.md

@@ -0,0 +1,2 @@
+### Fixed
+- WK crashing when changing the color of an empty segment group.