fix: prevent nil pointer panic in mention notification listeners

When a task bucket is updated without changing buckets (early return in
updateTaskBucket), b.Task remains nil. The TaskUpdatedEvent was then
dispatched with a nil Task, causing a nil pointer dereference in
HandleTaskUpdatedMentions when accessing event.Task.Description.

This adds:
- A nil guard in TaskBucket.Update to skip event dispatch when b.Task is nil
- Nil checks in HandleTaskUpdatedMentions, HandleTaskCreateMentions,
  HandleTaskCommentEditMentions, and UpdateTaskInSavedFilterViews
- Tests verifying the handlers gracefully handle nil task events

Closes #2351

pkg/models/kanban_task_bucket.go

@@ -239,10 +239,12 @@ func (b *TaskBucket) Update(s *xorm.Session, a web.Auth) (err error) {
 		return err
 	}
 
-	doer, _ := user.GetFromAuth(a)
-	events.DispatchOnCommit(s, &TaskUpdatedEvent{
-		Task: b.Task,
-		Doer: doer,
-	})
+	if b.Task != nil {
+		doer, _ := user.GetFromAuth(a)
+		events.DispatchOnCommit(s, &TaskUpdatedEvent{
+			Task: b.Task,
+			Doer: doer,
+		})
+	}
 	return nil
 }

pkg/models/listeners.go

@@ -245,6 +245,10 @@ func (s *HandleTaskCommentEditMentions) Handle(msg *message.Message) (err error)
 		return err
 	}
 
+	if event.Task == nil || event.Comment == nil {
+		return nil
+	}
+
 	sess := db.NewSession()
 	defer sess.Close()
 
@@ -390,6 +394,10 @@ func (s *HandleTaskCreateMentions) Handle(msg *message.Message) (err error) {
 		return err
 	}
 
+	if event.Task == nil {
+		return nil
+	}
+
 	sess := db.NewSession()
 	defer sess.Close()
 
@@ -422,6 +430,10 @@ func (s *HandleTaskUpdatedMentions) Handle(msg *message.Message) (err error) {
 		return err
 	}
 
+	if event.Task == nil {
+		return nil
+	}
+
 	sess := db.NewSession()
 	defer sess.Close()
 
@@ -541,6 +553,10 @@ func (l *UpdateTaskInSavedFilterViews) Handle(msg *message.Message) (err error)
 		return err
 	}
 
+	if event.Task == nil {
+		return nil
+	}
+
 	// This operation is potentially very resource-heavy, because we don't know if a task is included
 	// in a filter until we evaluate that filter. We need to evaluate each filter individually - since
 	// there can be many filters, this can take a while to execute.

pkg/models/mentions_test.go

@@ -22,6 +22,7 @@ import (
 	"regexp"
 
 	"code.vikunja.io/api/pkg/db"
+	"code.vikunja.io/api/pkg/events"
 	"code.vikunja.io/api/pkg/notifications"
 	"code.vikunja.io/api/pkg/user"
 
@@ -94,6 +95,18 @@ func TestFindMentionedUsersInText(t *testing.T) {
 	}
 }
 
+func TestHandleMentionsWithNilTask(t *testing.T) {
+	t.Run("HandleTaskUpdatedMentions should not panic with nil task", func(t *testing.T) {
+		events.TestListener(t, &TaskUpdatedEvent{Task: nil, Doer: nil}, &HandleTaskUpdatedMentions{})
+	})
+	t.Run("HandleTaskCreateMentions should not panic with nil task", func(t *testing.T) {
+		events.TestListener(t, &TaskCreatedEvent{Task: nil, Doer: nil}, &HandleTaskCreateMentions{})
+	})
+	t.Run("HandleTaskCommentEditMentions should not panic with nil task", func(t *testing.T) {
+		events.TestListener(t, &TaskCommentUpdatedEvent{Task: nil, Comment: nil, Doer: nil}, &HandleTaskCommentEditMentions{})
+	})
+}
+
 func TestSendingMentionNotification(t *testing.T) {
 	u := &user.User{ID: 1}